There are a variety of exercises you can do that are designed to increase muscle strength in areas that don't traditionally benefit from cycling alone, but when stronger can help your cycling. These include squat jumps, lunges and any number of upper-body exercises, such as single-arm pulldowns, pushups, pushing a "weighted" sled, etc.
One of the main benefits of these exercises is that they not only increase your strength, but your flexibility. The latter can be very helpful in cycling.