Things you can do that are fun in a self-abusive kind of way:
FastPedal - warm up in zone 1, then pedal as fast as you can in a low gear and still stay in zone 2. Try for 115-130 cadence. Have to have a computer with cadence like Cateye Astrale for this. Do it for 15-45 minutes continuously. I find that 42X25 is about as low as it is reasonable to go.
Endurance ride - A little harder than zone 2 for as long as you can stand it.
Recovery ride - just zone 1
Endurance recovery ride - just zone 2
Tempo - let some air out of your tires and pedal in your biggest gear at a cadence that puts you solidly in the upper part of zone 3. Cadence of 70-75 would be good. Do a couple of 20 minute intervals or even more as you like. Once a week.
One-legged pedaling - 2 minute intervals on each leg, then spin with both legs 2 minutes. Do some at 50 cadence and some at 85 cadence. Use enough gear so that you are really killing yourself for the last 15 seconds of each one-legged interval. 15-45 minutes of this. Because you only use one leg, your HR should stay in zone 2. Prop the lazy leg in the frame triangle.