[Mieann]

I voted for the ballet outfit because it looks really cute!

Mm. I hope you are buying the entire set, including shoes and white stockings.

Here's a suggestion from me. Find somewhere that is slightly risky/public that you think you can change. (Maybe a locker room? I don't know).
Change in this order:
Bags on the chair
Sit down and take off shoes, then socks
Stand and take out your costume. Put it atop your bag.
Take off your bottoms, including underwear
Put on the costume
Remove bra if you are wearing one
Slide your hands under your top and pull the straps of the costume up your shoulders and wear it. It looks like the tutu skirt is sewn on.
Put on your stockings.
Remove your top.
Fold your clothes and keep them neatly on your bag.

As for what you can do in it... Mm, maybe you want to do some ballet stretching? Leave the room and do a quick stretch before running back into the room? Or maybe twirl your way back?