Don't just reward players for relaxing, make relaxing interesting. Let me explain.
First, don't just reward players if they spend time on leisure. Without suggesting there might be an alternative, ask them what their character does to relax. This might require some finangling so they aren't tempted to head off for the next adventure. Have the village hold a festival, if necessary - and do it between adventures. Yes, it's not a subtle method, but you should only have to do it the first time - because you'll be rewarding them for it, and if your players are anything like mine, they'll want to do it of their own accord.
Whatever reward you usually give out for roleplaying, that's what you should give here. After all, your goal in wanting players to define their leisure activities is to improve the verisimilitude of the game world and immersion in character, and that's same purpose and definition of roleplaying rewards. Whatever argument you use to justify roleplaying rewards will apply equally here.
That said, don't just "reward characters for relaxing," either - Reward them for relaxing in a way that is fun. It may sound harsh, but your players only care about the well-being of their characters insofar as it enhances their own. That's entirely reasonable: The entire game exists for their (and your) entertainment. They have no motivation to let their characters kick back for a few hours unless they get something out of it. Fortunately, there's a simple solution:
Make deciding how their characters spend their leisure time a creative exercise. Many players enjoy defining who their character is, and what makes them tick. If you frame your question about leisure activities in those terms, your players may well leap at the chance.
After all, what someone spends their spare time and coin on says a lot about who they are: A thief who tosses a bag of gold on the tavern counter and goes from standing to drunk as soon as possible is completely different to a thief that sprawls out on a grassy hillside for an afternoon nap. For that matter, you needn't limit this to purely "relaxing" activities: An ascetic paladin who dumps a bucket of cold water over his head and spends three hours doing sword drills could be a very interesting character, indeed.
I've done this in my Pathfinder campaign (largely by accident, but that's another story). As a result, I've had a pair of elves who've started a fine dining society, a rogue who dresses in frippery between second story jobs, a half-orc who can't sleep comfortably without the smell of horses, a pair of thieves who bet on themselves in illegal boxing matches, a bard who's spent hours working on designing better advertising flyers, a wizard trying to convince nobles to invest in a new bank, and more besides. The party friar even married a peasant couple, once.
Of course, all this hinges on our players being the type to enjoy portraying a character. Still, even if they aren't it could make a nice change of pace. It's worth a shot.