At another school we used to go to we did game night and people would bring in their own game boards like chutes and ladders, monopoly or even card games and teach the kids simple things like WAR and Go Fish to Gin Rummy if they were up for it. That was free and easy, assuming you had enough people to come provide the games.
How about an Open Gym Night? Families come and play in the gym-think basketball, dodge ball, other "PE" type games. Maybe give them the opportunity to bring their own supper and eat in the cafeteria with PTO supplying the drinks. You could also have the library open for people who want to read or play quiet board games.
Besides the School Family Nights on PTO Today, are there any suggestions on an event to be held at night for students and there families that could be held without charging anything?