14th and 12th Joint Birthday Party Ideas?

I'm a 13 year-old girl going on 14 in just under a month. I have a younger brother who is going on 12 and we were thinking of have a joined celebration in the holidays with our friends. All up, there willl be around 10-15 boys and girls all ranging from ages 11 to 14.

I don't know what to do and I am hoping to send out invitations by Friday. I need desperate help on what I should do. I don't want to do anything too physical like laster tag or bowling, and I don't want it to be at home as I live too far away from all of my friends. I need to know what to do for me and my little bro's birthday. It has to be cheap, cool, and unique (not too traditional!) Any answers would be a huge help thankyou!