Edit: I goofed and didn't read that the question assumes no gym full of stuff. With that said, I still think the advice of breaking your workout up and including plyometrics in between and changing the motion and intensity of the bike ride throughout the workout is key to keeping it interesting.
For instance, one day do 10 minutes on the bike and 5 minutes off where
- 10 minutes on the bike is at a slower, maintainable pace
- 5 minutes off the bike is intense plyometrics
... and then the next day
- 10 minutes on the bike doing intense riding to the point where 10 minutes exhausts you
- 5 minutes off the bike doing brisk walking around the room and very very light plyometrics. You can even mix in some abs here too.
If your goal is to do cardio using machines or stationary for a substantial length of time and you find yourself getting bored while doing it, the best advice I can give is to constantly switch up which piece of equipment you use - either on a per workout basis or in-between workouts.
For instance, if you do cardio 3 times a week, get a two week rotation going where you hit all of these machines once:
- Stationary Bike
- Row Machine
Or, you could combine machines together within the same workout. For example, do 15 minutes each of three of the above.
For a twist on combining those cardio machines together, when you switch, spend 5 minutes doing very light plyometrics - even jumping jacks would suffice there.
On some of those machines you can even vary the style of motion you make. Some suggestions there:
- On the treadmill, increase the incline, ortry a slower pace while side shuffling or walking backwards.
- On the elliptical, vary the angle and try going backwards
- On a stairmaster, turn around and do it backwards
- Tweak the custom programs on each machine to do things like Hill Training or Interval training... or Random.
In short, the principle I'd suggest is to constantly switch it up. Doing the same motion on the same machine for 45 minutes at a time is just asking for monotony.