1. take 8 x 11 construction paper, roll into cone shape and staple. Attach pieces of tulle to point. Instant princess hat. make bforehand or as an activity at party.
2. Use upside down Ice cream cones (flat bottom) on a rectangle cake. They look like turrets on a castle.
3. Look for some disney princes CD. some have interactive songs that could be a party game or two.
4. Hang Butcher Paper with a light, uncolored form of a castle and let kids color it.
(I have no kids, but have been to too many princess parties to count.)
5. Shop on Disney.com for cheap princess items. Some are bagged together, but could be opened and used separately as favors, decorations, cake toppers.