An excellent exercise that will tighten your butt. In the video, I use a bench for this demonstration but if you do not have access to one, then you can easily do this with just an exercise mat.
Lay flat on the bench (or mat) and place the bottom of your foot on the mat with your knee bent. Your stationary leg will be straight out. Press up and squeeze your butt, then lower back down. After you finish reps for that side, switch and repeat on the other.