I have a set of folding allen wrenches that came with a belt pouch, strapped to my bike frame with some velcro. And adjustable spanner wrench, Craftsman Speed-drive socket screwdriver. And a tire patch kit.
It's true every bike will be a little different, but as you can see here pretty much anything you will be able to fix on the trail can be done with #3, 4, 5, or 6 metric allen wrenches, an adjustable spanner wrench, and a tire patch (or gorilla tape). Anything else is probably going to require you getting it home first to do any more extensive work.