humboldt'sroads

I've recently had to go to a one-bike quiver that's seen me go from three bikes to one over the last few months, and most recently, sold my CAAD9 for a 2012 CAADX 105. The reasons are multiple, but first and foremost I was always jealous of mountain bikers having the ability to ride arguably more remote, and much more fun terrain, though I was unwilling to give up the speed and 100+ mile ride capabilities of a good road bike.

When riding fire roads and trails I know that I should probably carry more than I'm used to when venturing out on a road bike and am wondering what others consider as their minimum checklist or carry list for riding away from roads. I only carry the bare minimum on road rides, and wouldn't be caught dead with anything on my back, though I usually don't figure I'm gonna break a spoke or chain, or flat more than twice on a typical road ride. I mostly will be riding 20-40 mile rides of mixed road and fire roads. I wouldn't mind eventually getting into longer more technical stretches up in the foothills or Sierras.

What do you carry? Maybe this is more appropriate for the MTB forum, but I'm not doing anything super core. I also figured in the MTB forum opinions would vary from "I only carry water" to those who take the whole damn garage. I'm looking for the "basic essentials" lists of other mixed terrain riders. Is a chain break, a few links advised? Kevlar spoke? Derailleur hanger? I don't quite have a feel for how much more likely I am to have a mechanical problem than I normally would on a road ride.

Also, maybe an obvious question for those riding cross or MTB for a while, but I have one road tire I love for all seasons. How useful is it to have 3 or more sets of tires for various conditions? I quickly realize having a set of wheels mounted with road tires will be all but necessary since this will also be my commuter and road bike, but I also realize my Schwalbe Sammy Slicks aren't good in mud or loose stuff.

Thanks in advance for opinions on the above or other essentials.