Non-Violent Means of Rebellion
What? Passive resistance? Can it possibly be, well, possible? Oh, yes. And it is wonderful. Peasants don't have to charge around smashing everything up in order to make trouble. They can resort to far sneakier means if they put their minds to it.
Denial of Service
Ah, there's a heading that's sure to draw all the wrong sorts of traffic. In this case, the peasants can simply refuse to serve the people oppressing them. They can withdraw their labour. Go on strike, even. While this isn't always an option in a comparitively primitive agrarian society – refusing to farm means there's no food on the table – the peasants can still deny service in small but unpleasant ways: an innkeeper could refuse to serve or house the PCs. Certain vital commodities could suddenly become unavailable.
Bad Service
Are the adventurers bad-mouthing the blacksmith? Oh, dear. What happens when their horses need to be shod? The village smith is hardly going to want to do the best of jobs for them now, is he? This is where another minor form of resistance can really screw the heroes up. A blacksmith could 'forget' to use all the nails he needed when fitting horseshoes. Beer could be weaker than normal, or spiked with something to render the drinker incontinent. Food can be spat in; soup, stew or broth can all be wanked in: all of you have had a crappy job at some point, and I'm sure you're all aware of half a dozen little ways in which you could get your own back against crappy customers.
It's Good to Talk
There's not much to do in a village if you want to relax: work takes up much of the time, and the people have little use for books. There's no theatre, and not much in the way of music. That leaves gossip as one of the main leisurely pursuits. If one peasant's wronged, it's almost certain that he'll make sure everyone else knows about it.
Given how quickly bad news travels, it's worth jotting down a few bits of gossip, however trivial or unimportant for every village the PCs enter. Even if the gossip has no bearing on the adventure, a small detail about how Old Turgot is a complete bastard for not returning the rake Derek lent him could develop into a future adventure hook. After all, people always want allies when they have disputes.