What should the bride wear to the rehearsal?

This depends on what kind of rehersal you are having. If you are having something more relaxed then something 'Casual Dressy' or 'Casual' dress codes are appropriate. However if you are after something a little more 'Blacktie' or 'Cocktaiil' then you can specify this as well. 

At the end of the day it is your "before the day" day, so it is your call as to how fancy you want to make it. The only thing to check would be dress code restrictions on certain venues.

Wedding rehearsals are typically not formal events so casualwear is just fine to wear for your rehearsal.