Firstly a big hug. This age was really hard for us too. Ds had no friends, his siblings found him difficult and most activities were blighted by meltdowns and tantrums. But now, 7 years later Have just spent a lovely evening with my children doing lip synching to Joseph and Technicolour and playing Articulate, we have all been crying with laughter and Ds has been to Football match with dh. So clearly things have got better and they will for you too.
Swimming pools were for us a good place, but for some ASD children they are a sensory nightmare. Very noisy, very confusing. Maybe it just wasn't suitable for him on a busy Saturday, combined with the usual stress to poor parent of having to get child undressed dressed and possibly self dressed and undressed too (we were lucky our swimming pool allowed parents to stand by poolside fully clothed when kids went in as it was very shallow kids pool)
Its fantastic that your son enjoys his bike and scooter. Maybe even in the winter there can be a place you could take him for bike rides away from too many judgemental people? I think even on a wet cold day it is worth just getting out, possibly to same place over and over, familiarity does help with some activities and cuts down the anxiety and defiance that goes with it. Simple things like snacks and crisps and sucking drinks can calm slightly all over the place 7 year olds in a public place. One of our issues with ds2 was toileting, he had very little awareness so would suddenly need to go when a place with no immediate access to loo - this was very stressful for us and made me feel very "nightmare" - looking back there was a lot of hanging on and acting out because of this, so might be another thing worth thinking through in advance.
Social stories also helped us with transitions, talking about things before we did them, planning them quite explicitly, making contingency plans, making things easy for the adults so they were less stressed and able to cope better with inevitable meltdown (ie ready meals, cafe food, instant snack lunches)
Dancing at home is another thing my children enjoyed, and was helpful on cold winter afternoons. Sleeping bags, baths in afternoon are other ideas. Out of Synch Child has Fun has a good many ideas.