DS1 (6) already does a few things - dancing, football, swimming lessons (in season) and now Tae Kwondo. Next year I'd like him to take up a musical instrument, either piano or guitar and he does want to do that. DH wants him to have tennis lessons but maybe not for another couple of years yet - and I'd like to let him have riding lessons too, but we do realise that he needs time off as well, so I don't know how we'll fit it all in!
When he can swim, probably the swimming lessons will drop, unless he really wants to do them - I don't care if he can do all 4 strokes, so long as he can swim safely and functionally.
He may want to give up the dancing as he gets older, which will make room for other things (I'll be a bit sad though)
The Tae Kwondo is currently for discipline, self defence and stability - he may or may not choose to continue with that (he's only just started so we'll see)
I did think about foreign language lessons but really, I think it's too much for DS1
DS2 will pretty much follow in what DS1 has done, unless he doesn't enjoy it, in which case we'll find other things for him to do.