I can tell you that whatever you do, you must be organized and do not just "wing it" It will be a disaster. You have to have a plan.
I can tell you what my 13 and 14 year old would love. If you can get hold of or borrow a few DDRs (Dance Dance Revolution) My girls play on these forever! When they have their friends over, its all they do.
How long is the party? Will this be a mixed party? boys and girls? I thought of the roller rink....they have boys only, then girls only, then boy asks girl, then girl asks boy. Just make sure the music fits the genders. Then take small breaks in between the music for the fun songs.. The chicken dance, the hokey pokey (yes, they are corney but even adults still play along) You can even host a "So you think you can dance" contest.
I'm not favorable to couple dancing, so if it were me, I would turn the gym into a giant gameroom. Call all your friends, family and rental facilities and get a few ping pong tables, a volleyball net, have basketballs out if there are hoops in the gym. I would let the first hour be free time, then do the singing, cake and presents, then another hour of free time. If you have more time, do relays. Our favorite was fill two brown paper sacks with different foods, each in a little baggie. (4 marshmallows, 4 caramel candies, a jar of baby food desert, 6 crackers, gummy worms,etc) Set up two chairs with one bag in front of each chair. Have two teams stand across from the chairs. When you say go, the first person on each team runs to the chair, reaches in the bag and grabs a baggie. THey have to eat it all and swallow it before running back to the next person in line. The first team to finish wins. You can decide whether or not you want to give prizes.