That is, characters doing frivolous stuff like drinking too much ale at the tavern and getting drunk. Spending gold to see a minstrel show. Gambling and brawling. Patronizing the local house of ill repute. Splurging on a lavish meal. Activities which real adventurers or soldiers would do in real life after surviving a harrowing adventure, but which a D&D player hoping to maximize XPs or gold earnings wouldn’t consider. By “encourage” I mean mechanics that use either a carrot or stick approach to prod PCs.
For me in particular, maintaining suspension of disbelief in an adventure is important. That means characters aren’t just monster killing machines but do believable things that add color to the adventure. I’m not saying players need to roleplay all of these activities; a simple die roll to check if a character is wise enough to steer clear of the whorehouse would work. If he doesn't make the die roll he's out 1d8 gp. (Although if the PC is a country bumpkin who's never been to the big city, maybe there would be an XP reward for doing crazy shit at least once?) I’m limiting the discussion to adventure themed activities that are potentially fun – so for example having a picnic or falling in love and getting married probably isn’t something players want to simulate in a game even if it is something “real people” might do.
I’m interested in D&D v3.5 and Pathfinder rules, as written or homebrewed, but rules from other versions or even other RPG systems that can be easily ported into D&D are welcome. I suppose I could come up with rules on the fly, but I'm hoping someone's already thought of this and has tried and true rules that are consistent, intuitive, and of course fun. :)