This all depends on your goals, your likes and dislikes and your level of fitness currently, but here's some ideas:
Goal: Lose weight Fitness Level: beginner to intermediate Try some Zumba - doesn't matter if you're a bit uncoordinated as it's the effort that counts here.
Goal: Lose weight Fitness Level: intermediate to advanced Try spinning. It's quite intense and get the heart rate really going, so needs a decent level of cardio fitness to start with. Make sure you get the right class though as they do vary massively!
Goal: build and tone muscle Fitness level: intermediate to advanced Try CrossFit. Great to help you jump higher, run faster and improve overall sports performance
Goal: build and tone muscle Fitness level: beginner to intermediate Try a kettle bells class. A total body workout that helps strengthen muscles and improve endurance
Goal: performance and endurance Fitness level: intermediate to advanced Try Boxing boot camp. For full cardio, strength and stamina. Tightens your core and improves you speed and agility.
Goal: flexibility Fitness level: intermediate to advanced Try a Barre class. Dance conditioning using a ballet- bar. Limber up without bulking up
Goal: improve overall health Fitness level: all TryYoga. Great for flexibility, posture and good mix of mind and body conditioning. Different styles of yoga to suit all levels