Cheap and easy party idea:
We recently had a Scavenger Hunt party at the mall for our 9 yos birthday. We did it at Clackamas; any mall would work.
We split the kids into teams of 3 or 4, each team with an adult or teen. There were 6 clues (eg. "This is Granny's favorite candy shop. The logo is black and white. Find it, and write down the price of dark chocolate caramels".) The kids would have to read the clue, figure it out, find the store and write down the answer. Each team had the same 6 questions, in a different order, but they all had the same last question, so they all ended at the same place. Then we had cake.
Each team had clues, pen, etc. in a ziploc bag. The adults on the team couldn't help, only keep them from getting lost. Some clues required them to unscramble the location name, some to do a little something, some to get information or a business card. This kept them from just figuring out the answers without going to the stores.
My eldest daughter went to a Sweet 16 party where they did the hunt downtown all over fareless square. Both ages loved it!