If it is the activity itself which is boring, some other options might be a rowing machine, a versaclimber, a regular stationary bike (which shifts the emphasis to leg only), a stepper machine, or an elliptical machine. Some people suggest doing prowler sorts of pushes by walking on a dead treadmill - forcing the shut off motor to turn provides some resistance. (Probably don’t want to try this on a good treadmill, as forcing the motor to turn might damage the electronics.)
You could also experiment with non-traditional modes of cardio: kettlebell swings or snatches for high reps, barbell complexes, other kinds of resistance exercise circuits done for high reps with short rests. Depending on what you are after (central vs peripheral adaptations), these might be an acceptable alternative.
You could also try switching between two modes of HIIT just to reduce the boredom factor: use the bike once a week, then do something else on for the second workout. Likewise, you could try varying the type of HIIT program you are doing. Use lots of short work intervals one session, then longer and fewer work intervals on the next workout. Martin Gibala has book out which has a wide variety of HIIT programs you could experiment with.
I personally have done HIIT with a rowing machine, a stationary bike, and an elliptical machine:
I found that it was harder to get my heart rate up with the rower, just because there is more technique involved. However, it is more interesting than a bike or elliptical, just because there is a skill component that you can work on.
For me, the stationary bike was the simplest and most reliable way to get my heart rate up, but since it is all leg work, you really have to grind those legs hard.
I currently use an elliptical machine, because I have one at home, and no longer have a gym membership. I have come to prefer that over the stationary bike because it is load bearing, and the stress is spread out over arms and legs. But subjectively speaking, I do feel like I have to push myself a little harder on that machine to hit my heart rate targets.
My overall experience, though, it that HIIT is never really that enjoyable, regards of how I do it. It is something that I just have to grind through for whatever benefits it provides.