I think a really nice ballet bag would be cool. You could do a little research on what other dancers typically keep in their dance bags an fill it with goodies! Like a cool water bottle, a resistance band, a cute sweat towel, hair accessories, extra tights. I'll edit my comment with some ideas when I have a second.

Edit: Nice gym bag, Resistance band, My favorite way to get a perfect bun, Good sweat towel, Spill proof water bottle(VERY IMPORTANT!), Tights.