I teach a circuit training class every week or two. I was getting bored with the usual set up of various stations around the perimeter of the room. Everyone knew what was coming up next, bicep curls, lunges, jacks, etc.
Now, to make things a little more interesting and fun, I still place steps around the perimeter of the room, but instead of the participants knowing what they'll be doing at each station, I place index cards with each excercise face down in the middle of the room. Each person, when it's their turn, runs to the middle of the room, turns over a card, and then we all have to do that excercise for 1 minute (or however long you choose). It really keeps them on their toes, not knowing what they'll do next.
Here are some excercises I put on my index cards:
...and many more.
My class really enjoyed picking their own class.