It's time to get some ballsmedicine balls, that is. I've found that incorporating medicine ball work into strength- and power-training programs can really help to break the monotony of traditional weight-lifting routines. But don't just do sit-ups with a medicine ball, go outside and start throwing it around. Now, you have to be careful throwing a weighted ball, so please don't get too carried away, but ten-yard chest passes or overhead tosses are a great way to build strength and power, as well as maintain muscle definition. If you don't have a friend who's confident in their ability to catch a thrown medicine ball, throw it against a wall instead. I'd suggest investigating the wide variety of other medicine ball exercises you can do either solo or with a partner. It's fun, it's challenging, and it can be as hard a workout as you want to make it.
The other piece of cross-training I'd suggest is plyometrics. These exercises are mostly used to develop power and speed, and involve jumping and bounding. A simple introduction to plyometrics is stair bounds. Start at the bottom of a long set of stairs and instead of climbing step-by-step, take two, three, or even four stairs in a single-leg bound and then continue bounding up the entire flight. Plyometric exercises also include single- and double-legged jumps on and off boxes, over a series of cones, or over lines on the floor. Plyometrics will do more for your lower body and core than your shoulders and pecs, but they're a great addition to monotonous weight-lifting programs and can be very useful for helping athletes break through plateaus with lower-body lifting exercises.