DD chooses. Once we agree to something, she has to remain committed for whatever the appropriate duration is (some classes are paid in 3-month increments, full sports season, etc). She's free to change that out for a new activity once the commitment period has passed.

Here's a catalog of things she's tried so far... asterisk means it's still active:

Dance
Gymnastics
*Cheer and Tumble
Guitar
Soccer
*Drama
*Girl Scouts

Swim classes are something I see here frequently mentioned. We've got that covered at home.