hey i have conscidered doing the same thing (coed party) and what i had thought about doing since i'm having his sis as one of my BMs (she's 12) was going to the go cart tracks. they also have a lazer tag thing so i was thinking boys vs girls!!!! keep it competitive, but also have fun with our friends and such. maybe as suggested after it starts getting late, take those underage home and head out for some late-night grub at a restaurant like chilis or fridays where those who wanted to drink could, but no one felt obliged to do so.
Hope this helped and GOOD LUCK with all the planning