D.G.
One of the funnest b-day parties my 10 year old went to was a scavenger hunt and then a nerf gun "war". They divided into 2 teams - red & blue and whichever team finished 1st won. Then after cake they did a nerf gun war. The teams each took a side of the room and everyone had brought their own nerf guns. It was great ! The boys had a wonderful time. Another idea is a video game contest if you have a console or 2.