I usually DO announce the new mechanic unless there's some reason beyond "guess what the GM is thinking" to keep it a secret. As far as XP goes, I give out XP for defeating monsters and recovering treasure. If the players don't engage in those activities, they get no XP.
That might sound harsh, but I'm upfront about it from the very beginning and have gotten no complaints. If the players aren't "adventuring" then they aren't "adventurers". Giving out XP for NOT adventuring would result in most NPCs being super high-level characters rather than level 0 nobodies. Gaining XP (and, axiomatically, Gaining Levels) requires risk before reward. Roleplaying is fun, but shouldn't in and of itself grant XP unless it involves some sort of risk. Real risk. Not risk to one's ego or reputation or whatever other intangible-thing-that-doesn't-matter your players might argue for in order to get some free XP for playing it safe and not adventuring.
Just my two cents.