When I go to the gym, I either use the cross trainer or semi recumbent bike for my cardio work. I usually try for 30 to 45 minute sessions, and the intervals are either cadence based or resistance. So, on the bike I may warm up for 10 minutes @ L11/12 @ 100RPM, then do 6/8 X 1min @ L16/17 with a 2min rest in between, then do a gradual cool down. If I was doing cadence based, then the level would stay at 11/12, but the interval RPM would go from 100 to 120/130. I also do a "ladder" routine, where the warm up is followed by a gradual increase in the levels 12/13/14/15/16/17 for minute each, then rest for 3 minutes and repeat 3 to 4 times, which can be quite brutal.
I have often thought it would be nice to have a shed with a big telly and watch cycling classics whilst you train.