You might consider some math that she won't encounter elsewhere. We're doing some Beast Academy-- or do some fun math apps (they have seriously enhanced my son's math calculation skills).
Agree about staying away from mainstream education in the summer.
I think if your daughter is happy with her summer, you may not want to rock the boat too much-- as I said, math as game online is a stealth way to improve skills with it seeming like school.
My son swims and I'm also thinking of photography for him next summer. Swimming and especially a swim team is an amazing sport- my DS has learned a lot about tenacity on swim team. He also liked chess camp- and nothing with enhance self control like chess. A lot of life skills learned across a chess board.
