Gymtime,
Can you swim and do you have access to a pool? I was in the same dilemma as you, as I usually waste my legs pretty well on leg day, and am sore for at least two days afterwards (especially my calves). I didn't want to give up my cardio day, so for the first time, I went swimming for cardio the day after my leg day. Worked like a champ!! Now, I'm not the world's greatest swimmer, so it was a bit of a struggle to do all 20 minutes without drowning, but as long as I switched strokes often, I was able to make it through. got a pretty damn good cardio workout, and my arms, chest and back were actually friggin pumped when all was said and done. Leg's were no worse for the wear, since I used them very little during the swim. It was a little weird since my apt. complex has such a small pool, and I had to wait for the kiddies to get out of it later in the evening, but if you have access to a pool, I highly recommend considering swimming as an alternative after leg day.