I'd be inclined to skip the flash and bring a great tripod.

I spent ten days backpacking in New Mexico last year with a Canon 40D and 15-85mm, 50mm, and 70-300mm lenses. I brought a LumoPro LP160 flash, but left it at base camp, and I rarely missed it. Your T3i has a pop-up flash that is actually better than nothing at all. I used the flash on my 40D on a handful of occasions (below). In all cases, these were "fill flash" situations, and I think it's fair to say that these are shots to document the trip, rather than anything I'd expect to win awards.

If I were to entertain the thought of carrying a strobe, I'd personally be pretty strongly inclined to bring a set of remote triggers, too, because there were definitely cases where I'd have been able to use an off-camera flash. Just as I'm tempted to bring triggers, though, you'd have to consider whether you're going to bring extra batteries for the flash (maybe not a big deal if they double as spares for flashlights or something else), and other flash modifiers. The point is that it sort of opens the door to a whole other pile of stuff to haul around.

On the other hand, there's no way I'd leave home without some sort of tripod. I brought a Trekpod, which ultimately bought the farm somewhere along the way, but you could consider other options, too, including something like a GorillaPod. Just be sure you've got a way to steady your camera for long exposures -- I have a feeling you'll find that at least as valuable as a flash.