In the bike's defense: as a runner, spinning on the bike is an absolutely awesome to maintain cardio-fitness after a running injury.
Squats were used by Soviet marathon runners for training in winter, when it is too cold outside to run. They squatted for hours.
One my squat is approximately 2.5 second. 5 minutes amounts to 120 squats. 30 minutes - 720.
I think you'll appreciate the power of 720 squats. ;)
Squats are pretty variable. You can put your hands behind your back, fold them on the chest, actively wave them helping squats - all those variation will give you different feeling.
Running isn't too bad either. Go barefoot or with something like Vibram FiveFingers if you care about your joints at all.
Also---in addition---consider getting a pull-up bar. Pull-ups [1] are a nice functional exercise for your upper body.
[1] And press-ups, but they don't require any equipment.
I have seen this work wonders on countless people.