I like simple but effective exercises. I do quite a lot of variations of Squats and Lunges:

Sumo Squats, Squats with small dumbbells (you can add a military shoulder press to this exercise), Jumping Squats, One Legged Squats...

Lunges, Crossover Lunges, Staying in the Lunge position while doing various dumbbell exercises with your hands, Jumping Lunges...

I also do hamstring curls on my bodyball (you can do them one-legged for added resistance), thigh raises, Side Plank with leg raises, Wall sit - exercises etc.


And you can always choose exercises that strengthen the whole body, like Burpees, they do a lot for your core strength and the little jump at the end adds some leg strength too.