multiple things if they would like. However, with anything they sign up for, I have to teach them that they can't sign up for something unless they are able and willing to fully commit.
For example, my daughter does competitive dance. This requires her to be at the dance studio for a few hours on Mondays, Tuesdays, and Thursdays. She is also a very talented soccer player, but while she may be talented to play competitive soccer, I know there isn't enough time in her schedule (nor money in my bank account) to allow her to do both. So she has basically had to choose between either competitive dance and rec-level soccer or rec-level dance and competitive soccer (and she has chosen dance).