DD is almost 4, and is (perhaps only through my rose-tinted pfb spectacles!) unusually agile and physically able. Her main strengths are balance (very noticeable a year ago, but not so different to other children now), upper body strength, and agility. And she just seems to 'get' how to use her body - and loves doing that 
Whilst I don't have sport-related ambitions for her, I don't want her to miss out on something life-enriching because she didn't start early enough, especially because we're not a remotely sporty family, so this doesn't come naturally or automatically to us.
So what activities and sports have a 'start-by' age? That might be due to the nature of the activity, or because it gets so booked up. And is there anything else I should make sure I do for her?
She already does pre-school gymnastics which she loves (a perfect example: if you haven't started in pre-school you simply won't get a space around us).
She also does swimming. And we spend many, many hours in playgrounds!
Would some kind of ball-related activity be good? A friend does tennis lessons - is that a good idea, or is something more general better? While she enjoys catch and kicking balls around, it doesn't seem to be a natural area of strength for her. That might be because we haven't done much of it with her though.
She has been asking for ballet lessons for a while, and we did a trial about 6 months ago, but she didn't really 'get' it so we didn't take it up. She does love watching dance and twirling around to music, and is still asking for lessons though, so perhaps that was a bit mean of me!
I was thinking of trying to find a tap or modern class initially, then possibly do ballet later if she turns out to love it. Does that sound reasonable or not?
Thank you so much for any advice. I'm sure I'm worrying over nothing, but I just want to make sure I don't miss something important.