Relaxation? Productivity? These don't go together... normally. If you just want to put butt to couch and think of nothing, that's fine. On the other hand, if you have a few drops of energy in reserve and wouldn't mind spending them while you chill, that's possible too.
Alyse Kalish over at The Muse has compiled a list of activities that combine relaxation with doing stuff. Along with the usual suspects such as reading or listening to a podcast, she includes more productive pastimes:
Write: Start a blog, write an article and post it on LinkedIn, journal. Don’t worry about composing something spectacular—just get those thoughts on paper.
Cook: Or bake! Find a new recipe and try it out—and if you bring your creation into work I guarantee your co-workers will love you.
Solve a Puzzle: Challenge your mind a bit by working on a physical puzzle. Or, play solitaire with yourself. If you have a roommate, play a board game.
Of course, if you feel you'd have trouble chilling out while doing any of the above, that likely means you really do need a few hours of nothing.