I always bring my smallest Flash, my Canon 430EX, for just the things you mention. But I'm a digital imaging pro and photo nerd, so others' mileage may vary.
"Who would use a reflector while backpacking anyway?" Anyone who doesn't want the light to SUCK! :-) (kidding!) But seriously, after anyone reads strobist.com or gets some direct-flash experience will find that direct flash is awful, horrific, and worth avoiding. I always bring a few 3x5 cards for catchlights in people's eyes and yes, if you carry a chrome umbrella, well, you're gonna be able to take some awesome shots...but you'll also have to hold your camera, and your 'brolly, or you're packing a tripod and you're as bad as I am and then we're not UL weenies anymore, are we? :-p
There are times when my beastly camera feels so heavy that I just can't bear to take a flash with me...and if I'm hiking with someone else, well then - POOF! Instant photo assistant. In such cases, I carry items to still modify natural light to supplement the classically painful dark-trees-blown-out-sky kinds of problems. Also perfect for getting rid of those face shadows from those wearing billed hats.
- Tin foil for sharper fill lighting, easier to aim for newbie "assistants." Bring lots.
- Heck, bright white glossy printer paper (a few sheets). Like tin foil, but much softer quality of fill light.
- Tape and tissue if someone must use direct pop-up flash from a Point & Shoot camera; loses a stop or so but softens the light when applied over the pop-up flash
- That chrome umbrella might just reflect enough light on its own to act as a fill light opposite the sun.
I find that scrims and other stuff to soften sunlight need to be too big to be useful, and impossible to set up right; go for lightening the shadows instead of dampening the light source.
And, finally, since binary data weighs nothing, I bracket like hell so that I can combine multiple exposures in Photoshop later, so that I can fill in deep shadows in post rather than carrying a flash.
I'd best stop writing lest this become painfully long. Hope this spurs some thinking.