Hi Tina!
I'm no expert, but I'm thinking that maybe any endurance style, full body workout would work as a sub. Do you have Push/Pull and Supersets? How about Muscle Max or Muscle Endurance? Good luck and have fun!
ETA: OMG, I'm such a loser. Obviously you don't have Push/Pull or Supersets since you're asking for a sub for Supersets!!! Dear god, it's been loooong day. Sorry.
Anyhoo, like I said, any endurance style, full body workout will do. I'm sure others will have some great ideas....
Since Supersets is a not as taxing as some of Cathe's other full body workouts (and therefore, right up MY alley! ) you might try subbing a total body sculpt premix of BM2, LIC, DM etc. You could also do HSCh and fast forward thru the cardio. Just a thought.
Jonahnah
Chocolate IS the answer, regardless of the question.