Not only does biking give you a great cardio workout, but when you ride a bicycle, you're working almost all the muscle groups in your thighs, hips, and butt. A 140-pound person can burn about 325 calories during a 45-minute bike ride.
For an effective 30-minute workout on a stationary bike, alternate between 3 minutes of riding at 70 to 80 RPM and 2 minutes of riding at 100 to 110 RPM.
You may also want to try taking a spin class, or riding a bike outside. To work your glutes even more, increase the resistance by standing up.