Everyones Favorite….2×20
The time honoured 2 x 20min intervals (5 mins rest between intervals) at FTP are a good session that fits in an hour
Also I like 7 x 4min intervals (1 min rest between intervals) at above FTP.
FTP is functional threshold power, and is basically the maximum power you can sustain for 1 hour (we are talking absolute max not just a hard pace)
Do the bikes have power measurements? I find power is a very good way of keeping the pressure on for this type of interval, as HR will tend to increase gradually over the interval for the same power output.