If you've got some time (lunch break not quite over?) and aren't worried about strange looks from your cube mate, why not try these no-equipment-necessary, bun-tightening Standing Booty Kicks?
Stand tall and place your hands on your hips. Put all the weight on your right leg, keep both legs straight and lift your left leg directly behind you. Let your glutes do all the work. Keep your left foot flexed and work on lifting your heel as high as you can without leaning your chest too far forward. Engage your abs to help you stay balanced. Lower your left leg so it's parallel with your right, but don't let it touch the ground, then lift it back up behind you. Repeat this move 20 times at a moderate pace, and then repeat these steps with the other leg.