Create a pirate ship, a photo booth, a corner market, or just a place to play hide and seek. This is an incredibly easy think to do. You can be as simple or elaborate as you want. It's amazing how kids will find ways to use their imagination by just cutting a few holes in a box.
You can find refrigerator boxes at your local appliance store. Don't try the large stores like Home Depot, Sears, Lowes, etc., they don't have boxes (although their delivery services might.) Any large appliance box will do. If you get several boxes, think about building a maze with them.
Here's one idea...the Sweet Shots Photo Booth. It is so simple and can be the hit of any birthday party.
Kids are able to go in individually or as a group, depending how many kids could fit in the booth at one time. Take several pictures of each of the party guests and then include the picture of the child in the thank you cards send a couple of days later.