I used a small plastic pet carrier (like the one you'd use for a mouse) and decorated it with military stickers I found in the scrapbook section of a local store and labeled it "House of Pain." In the "house" I placed 50 folded cards each containing 1 cardio and 1 strenghtening exercise along with the time/rep #. The rules are simple, everyone gets to pick at least one card and we go from one side of the studio to the other so everyone should know who is picking next. I give them a 5 second warning at the end of the strenghening portion and they have that time to run to the house and choose one card. I always pick the very first card, then place the house in the center of the studio. If they fail to pay attention (forget to pick) the "punishment" is to run 2 extra sets of stairs. No one yet has let it slip their minds when they are the next to choose.
After the last person chooses I give a warning during the strengthening exercise that the class will have a set amount of time to return all of the equipment to it's designated area. The "punishment" if they should fail the mission, is an extra 2 sets of stairs for the entire class for every 10 seconds past the allotted time. I started with 90 seconds the first time we did this and have decreased it every time since. I tell them to use the buddy system in order to beat the clock and they've always succeeded. Of course if there are more than 2 first timers to the class I adjust time accordingly.
My regulars groan every time they hear me say "House of Pain" but leave thanking me for a great workout.