I recently started doing Tabata workouts on a stationary bike. Interval training is an incredible approach if you can't invest much time. I can be in and out of the gym in 15 minutes and can feel the benefits 48 hours after a rough workout.

The challenge is that it can be incredibly difficult. I never regret going, but it can be hard to motivate knowing how hard that 15 minutes will be. I can barely walk afterwards. That's unfortunately a tough sell for most people despite its benefits and I find an excuse to avoid it more than I'd like to admit.