E.S.
For my son's 9th birthday we made marshmallow blow guns and it was a HUGE hit. It's super inexpensive and so easy to do. After we made the guns I gave each of the kids a huge bag of mini marshmallows and they had a hay day running around shooting marshmallows at each other. I also set up an ice cream sundae bar which they loved. Another idea is you could set up carnival type games in the house and have little prizes at each station. ie: attach a large piece of cardboard to a wall and staple balloons on the cardboard. Then have the kids use darts to pop the balloons. You could also do a ring toss with water bottles or a coin toss. If you did this I would recruit some of your friends or other parents to monitor the game stations. Good luck and have fun.