hmm.... lots of thoughts here.
Intervals on any type of cardio machine.
This is the way that I instruct intervals in my spin classes.
The basic idea is warm up 5 to 10 minutes, 8 reps of 30 seconds hard, 90 seconds recovery, 5 to 10 minutes recovery.
During the hard intervals, you should push your pace/tension/ intensity to the point where you are out of breath, can't talk, don't think that you can go on any longer. Then recover. You can do this on any cardio format, running, elliptical, etc. Outdoors on the bike you can do hill repeats to simulate the same thing.
As this has become easier, we have shortened the recovery times, increased the number of sets, increased the length of the hard intervals.
An example of a recent class is as follows:
30 sec hard, 60 easy
2 times (30 hard, 45 easy)
3 times (30 hard, 30 easy)
Standing:
3 times (30 hard, 30 easy)
sittting:
6 times (20 hard, 10 easy)
60 addtional recovery
45 hard 60 easy
90 hard 60 easy
3 minutes hard, done