Which activities are, in your opinion, worth it?

Swimming agreed.
Beavers I wonder about. It seems to me with the forest schools/ veg patches/school adventure trips that it doesn't provide much different.
Tennis is the one that stands out with DS. He started young but it was his choice & now has very good hand/eye co-ordination for many sports.

Swimming here as well. Tried in reception. Stopped. Have just started again in Y2 but have made effort to go swimming as a family in the meantime.
Beaver - took some perseverance to get into the habit, really enjoys it now. Good to have friends outside of school and they do lots of different activities.

Neither of mine was able to swim before 6. Getting them used to the water was the only benefit of earlier lessons.

I did singing / music groups because I rather liked them too. And introduction to ball sports from pre-school year up to age 7,because we didn't do that much at home.

Putting names on waiting lists for volunteer-run clubs like Rainbows and Beavers is a good idea, contact your local group.

(Both did Cubs - great leader - but for DD dance commitments got in the way, and for DS the transition to Scouts, which has much weaker leadership, meant too much chaos for his order-loving character to cope with, so he left)

Swimming.

Other than that, when they were end pre-school to end KS1 sort of age, the main thing in aimed for was a reasonable variety - so DS (sporty but many ASD traits at that age) did football, swimming, tag rugby, Beavers whikle DD did swimming, Rainbows, dance. Both also had loads of running round the park'-type free time.

In KS2, both took up a musical instrument through school, and to our surprise, that has becoe DS's main 'thing' in secondary. Both have also had 1 thing in which they became very immnersed - for DD it was, and is still, dance (10-13 hours per week now in Year 6), for DS it was football with a professional club's academy for a couple of years, but being dropped has returned him to a much greater variety, which he is very happy with.

Diving - being able to jump off a 5m board has done wonders for my 10yo DC's self-esteem and confidence.

Swimming, without a doubt, not just for its own sake, but because it opens up so many opportunities to them as they get older, if they are confident arounf water.

Then defintely Beavers/ Cubs/ Scouts / Explorers - my dc have so many skills gained from going to camps and meeting nights, as well as great friends, confidence, and overall, have just had so much fun.

If you can afford it then a musical instrument is a great thing to take up.

With most things though I'd say dont start the too young.

Mine have always swam because it's the only form of exercise I have any patience with and I love water parks.

DH was brought up by the sea, so keeping dry isn't an option in our house.

Other than that, you try things and keep the ones that work out.

DD1 sings fairly seriously and has been a Brownie - Ranger, she also does cold outdoor watery things.

DD2 did ballet until the teacher retired and loves gymnastics. She does lots of sport at school. She was a brownie and a scout, but the new leader/personalities at scouts didn't suit (she isn't mad in camping or hiking which didn't help)

Both have played instruments, badly and given up.

Drama is a great one - my Dd does a littlies class that's part of the youth theatre.

For us, the 'thank goodness we did that' activities have been music, drama and sailing.

Music and drama give them ready-made social stuff at secondary (though I appreciate you're a good way off that yet).

Sailing (hired boats, local reservoir, nothing fancy) gives antisocial DS1 a breath of fresh air and a distraction from teenage angst.

Oh, and DS2 dances (he started at 14 and wishes he'd done it earlier).

I have 6 year old twins. Dd does swimming, rainbows and dancing (ballet, tap and modern) 1 hour a week. Dis does swimming and boys brigade (which is like beavers but opportunity to learn a musical instrument I think). They chose to go to all of these, although I did encourage the swimming. Apart from that play dates and fun at home keep my 2 happy

It's not for every child, but I can't sing the praises of Beavers, Cubs and Scouts enough, for both boys and girls.

DS 8yrs has been in a football team since Y2, and was in Beavers, is now in Cubs.

DD 4yrs goes to ballet.

DD 6yrs goes to Beavers and is about to join a drama group.

I did try swimming lessons for DS in his pre-school year, but we gave it up when he was in YR as he was too tired.

I'm going to look into blocks of private lessons for my DCs, as was suggested up thread. The quality of the teaching and the progress made was always lacking when I used to pay for swimming lessons in the past.

I agree not to start swimming too young (unless it's a particular interest of yours of course). My dd didn't start until she was 4 but was very quickly in a class with children that had been swimming for over a year. I think they need a certain level of coordination and strength to progress which they just don't have when they are very little. 3 years later my dd swam 400m and is stage 5 so she wasn't disadvantaged.

Swimming

Beavers/Cubs/Scouts etc

A musical instrument they can play with others (orchestral or band) but would check out local opportunities before I chose.

Would also look at local sports clubs and choose the best run one or logistically easiest and then try that sport first.

Languages sound good in theory but the level of immersion needed to progress well isn't achieved by half an hour a week in my opinion. True kids learn languages faster when young, but that works for say a Polish kid at your school/nursery who is immersed in English every day, not an English speaker learning a few songs once a week.

Brownies
swimming
cooking club
chess club
gardening club
cross country

I'm going to go against the grain here, and if I had my time again, I wouldn't waste money on swimming lessons when they're too young.

I wasted hundreds of pounds on swimming lessons when DD1 was little, but she never really progressed. So I left it. Then, the summer before she started learning with the school, I paid for 10 private lessons. She learnt more in those private lessons, than in the years of weekly lessons / intensives that she had had at 4/5/6. When she did start learning with the school (after her 10 private lessons) she was then put into the middle group, and bypassed a whole load of children who had been having their weekly lessons for years. Also, the two children who were the best swimmers in her class, had never had swimming lessons either (they simply swam lots with parents).

Whilst I agree that it is important to learn to swim, I not entirely convinced on the merits of teaching children too young. I know I wasted hundreds upon hundreds of pounds, and with DD2 waited until she was much older. She learnt to swim in a few months, and again is a better swimmer than many of her friends who paid out ££££ for lessons from toddlerhood. She's having lessons with the school this summer, but I won't shell out on weekly lessons, I'll pay for 10 private lessons. In my experience, children make much more progress this way, and it saves money. (Both my children swim at least level 4).

Beyond that, follow your child's interests. My children are quite active, so I sent them to gymnastics. Both girls love that. They also dance, and they learn an instrument too. But that's what they've fallen into rather than having any great plan.

Swimming - mine have been every week from being babies and can give up or continue when they can swim 100m.
Otherwise with their interests, my 7 yr old does a Saturday group music lesson where he learns recorder, ukulele, guitar and keyboard, and an after-school free multisports session.
3 yr old does swimming and a music (singing, roleplay) class.
Once they get into Y1 (agree with prev comments about being tired in YR) there are often free or low cost activities offered by school. My 7 yr old has been able to try lots of things offered by the leisure centres in the holidays. He has done a couple of weeks a year of tennis camp in the summer.

How soon do I need go register DS?

Really depends on the area, some areas you could ring up and he could start next week (if he were old enough) others and you need to do it several years in advance.

I'd ask around local friends or else try and find a number to ring and ask about waiting lists.

swimming is the only activity that I planned to do with my dc's. Everything else was because they showed an interest when it was either offered or a friend was truing that activity.

From swimming dd1 did french club, ballet and went on to brownies, drama club, rugby and a bit of karate

dd2 swam for 6-7 years and went on to do running, triathlon, drama theatre, and cycling, she cycles now but different disciplines and is participating in d of e at school

I think they are all worth it depending on what your dc enjoy doing.
We personally didn't see any benefit in swimming lessons as dcs friends tended to still be going and their parents still paying many years later . They are all good swimmers though, went with school, during half term holidays, summer holidays, lessons of dh and friends etc. You don't need to pay.

Depends on the child. Let him choose.

Our Beavers have a waiting list of 12 months or so so I'd put your DS's name down when he starts school if you're keen for him to attend the troupe nearest to you.

Swimming in non negotiable in our house too.

Swimming and an instrument. We started swimming lessons after 3 yrs old (until they were confident and strong-ish), piano around 7 yrs old.