My favorite is a bike ride on my actual bike, but I'm sure I'm not burning many calories like that. It's a cruiser bike with a cute little basket.
At the gym, I do the weight machines for 30 minutes and the treadmill or stationary bike for 30 minutes. But my favorite EVER is yoga. I don't love the whole yogi buddhist vibe but I love the workout... gentle toning and increased flexibility.