body assistance

Does anyone know any extremely gentle abdominal exercises that can be done at home? I've been thinking about my various physical activity goals since hurting my toe and realizing how depressed it makes me not to be able to exercise--I started DDR again today and everything seems to have gone fine, toe-wise, so at least that's over. And also, said physical activity goals are more activity-oriented than exercise-oriented. Basically, "become able to do this thing [again], do it." Like, hiking, which I love more than life and almost never get to/feel able to do. Yoga again, some kind of martial arts again--a kind more on the "sport" end of the spectrum than the "practical self-defense skills" end, like the tae kwon do I did in high school with all those lovely forms to memorize and practice over and over. (Basically, if it takes place in Nature or if I can approach it like a videogame where the controller is my body, I will love it.) Swimming again, learning to sail, all kinds of things. Just generally getting myself to a state where I'm capable of doing this, which I read last summer and immediately declared I would get there in the next five years no matter what. One of the biggest reasons I want to move out of this city, aside from I grew up here and I'm sick of it and I want out, is so I can live someplace with elevation and less horrible weather and nature suitable for hiking that's less than four hours away. So that I can get in shape to do this sort of thing.

The biggest obstacle to doing...anything, at the moment, aside from my brain of course, is that I have no core strength. Absolutely none; I can't even sit up straight in a chair (not ramrod-straight--the good kind of straight like they teach in yoga) without using my back muscles exclusively, which of course starts to hurt after about thirty seconds. I've tried to do some very, very gentle yoga poses focusing on the core, but my back is so fucked up at this point that it always hurts, and I'm really scared of hurting it more. I'd love to start going to a personal trainer again but that's no longer an option because money, nor are classes--and anyway I don't think there's any group setting suitable for people with my degree of inability. It's really frustrating because I'm getting better at coordination and stamina with DDR--and actually I've always been pretty great at coordination, except when I'm just walking around not exercising and bumping into things--but I can't do more than about fifteen songs in a row no matter how good I feel, because my back starts to scream, because it's been holding me upright (or sometimes slightly bent over) the whole time. And I'd love to start lifting weights again but I'm so scared I'd put myself in traction using the wrong muscles.

So basically: I need to get myself some non-withered abdominal muscles, starting from absolute zero, without hurting my back and without outside help or any tools more expensive than, IDK, those rubber resistance bands. Any suggestions? :D?