Good ideas for a surprise party for my 13 year old friend?

I came up with the idea to take the train to a close by city and go eat out at her favorite restaurant with her two close guy friends and girl friends but lately she has been talking about how she wants a large party of around 20 girls and guys. Me and the girl I am planning the surprise party with have no clue what to do. We have a large finished basement to work with but not much cash. Ideas?