If you are doing the spa party thing....what is super fun, is getting one of those nail painting kits(available at hair supply stores) they are so cool!
My DD had her friends help her decorate the cake as an activity.
To go with the spa theme, a "goodie bag* idea I came up with was to go to Claires, and at the counter, they had clear pencil/makeup zip bags for $1. We got that, then went to the drugstore and bought little bottles of nail polish, nail brush, lip balm, and threw in some candy.Put a little in each bag for the girls and they loved it.
I have a friend whose DD gave each girl a nice little journal instead of a goodie bag, then the girls each created/designed/made bookmarks for them-each finished bookmark got put into a hat, and the girls took turns picking one out blind-folded. So easy to do with gel pens,stickers, and laminated sheets from Target. They even paper-punched each one and tied a silky ribbon through it to help keep place once they were using the journal.
When my DD turns 13, she said she wants to have a few friends over, and on their own, make and eat a gourmet meal and watch a movie.