Your backside. Chances are, if you don't look at it regularly in the mirror, you forget to work it. Having a strong back helps reduce aches and pains from living a sedentary life. A strong butt not only looks good, but it also keeps your lower back, hips, and knees healthy. To keep your entire backside strong while challenging your balance and core, try this multitasking move: single-leg deadlift with row.
Make sure you are adequately warmed up before doing this exercise — you don't want to pull a hamstring. Here's a short warmup that will definitely do the trick.