Have your friends over for dessert, snacks and good company. The object is to assemble lego sets from the manuals (or to make their own).  My husband and I came up with this one when we had to figure out how to quickly assemble approximately 100 sets of legos.  This was so much fun for multiple age groups. My friend had her kids come over to help and they brought a couple friends. All age groups seemed to equally enjoy the party. It's a great party for an adult to host because you have time to visit with your adult friends (while you assemble) and the kids are occupied for hours.  Plus it's really beneficial for brain development. Great time!