Well, you could always turn your rides into some version of circuit training by stopping every X miles and getting in a simple upper body workout of push-ups, and perhaps if you could find the right place/materials along the route, a bit more complex of a workout by doing presses with some laying-around object such as a rock (?). You could easily take along a resistance band or a TRX in a pack and hook up to a fence or maybe a small tree trunk and do rows, presses, and flies.