In terms of ideas, we have had:
Beavers/Cubs/scouts/explorers
Music lessons
Badminton (v popular)
Pokémon
Riding
Gymnastics
Rowing
Climbing
Sailing
The ones that have stuck:
DD (14, nearly 15): rowing. She now trains 5 days out of 7 and is normally at the gym the other days, so has dropped most others. Plus competing at weekends. Used to sail a few times a week and play netball, but rowing consumes her now.
DS (13): rowing, 3 nights a week with a competition on the weekend at the time of year. When no competition, climbing. I suspect the latter will be his 'thing' long term and his weekends will move towards that more, but he has rowing commitments this year.
Also, badminton with school friends, they meet every week and through the holidays. DH often plays with him too. He likes music as well, but only wants in school lessons at the mo.
DS (7): Beavers, cricket (over the summer) and is trying his best to do karate. He's quite an anxious chap and is determined to do it, so for 6 weeks or so we have sat at the side of the class watching and waiting for him to feel confident enough to have a go!