Not quite sure what 8 yr old girls like other than Hana Montana :).
There is a Cinderella carriage in downtown St. Pete. It's beautiful and I can't imagine any girl turning that down. Shoot adults love it. Anyway, go down there and get the number from the person driving the carriage. It usually rents for $50. We got it for $40 one night. If you do a tea party in the park before it gets dark. Small tea sandwiches, tea cups, etc. You can find tea cups on craig's list or thrift stores (they don't have to match!). After it gets dark, have the carraige pull up at a certian time and take the girls for a 30 min ride on the waterfront. Simple, reasonably priced and not alot of work! Publix sells Petite Fours that you can order from their bakery if you want to skip the cake and order them instead (they are little tea cakes and are Delicious and cute)