Joint 14th Birthday Party Ideas?

I'm turning 14 this August, but I'm having struggles thinking of ideas of what to do. My friend and I have decided to have a joint party, however as we live in the UK we don't want to do anything too weather dependent. Also, we have around 10 girls we would like to invite, but we don't want it to be overly expensive.

We have thought about a pizza party etc., but many of our other friends did that sort of thing for their birthday so we don't want to do something that's too similar to their party. I hope you can help with some ideas
Thanks.