EssaySnark says, follow your heart on this. Sure, strangers on a forum may have some interesting suggestions for you, but this is where you have the chance to make a contribution to the world in hopefully a meaningful way -- whether you get into bschool or not! EssaySnark gets somewhat disgusted at those who use volunteer work just as a way to stuff their app -- not that we're saying you're doing this!! You obviously have a history of getting involved, which is good. The adcom certainly looks for that.

It sounds like you'd be applying to bschool next year, so you're in a good position to find a cause that excites you, and roll up your sleeves and get involved. You should still absolutely do this even if you're applying to school this year -- but it won't count for much on your app if so. At this late date, the adcom may suspect that any just-initiated volunteer work is a ploy to buff the profile.

There are oodles of nonprofits and social ventures and any of them will appreciate your enthusiasm in helping out. Craig's List can be a way to find younger organizations; those often have the greatest need or opportunities to roll up your sleeves and dive in at the ground level with an important project. Or, since you're already working with Habitat, why not get more involved there? There's plenty of directories of volunteer orgs around on the web too to help match you up. The best stories for essays will be ones where you can show the ingenuity you brought to the table, the hard work, the resourcefulness, the people skills, or whatever (these are examples, not a discrete list) and MADE AN IMPACT for the group or its constituents.

Also though, don't get too hung up on this for the purposes of your application -- volunteer work counts for really very little at some schools, and at others, you'd need to make a pretty big contribution through volunteering in order for it stand out from all the other applicants. It's always nice to have something to talk about of course, and it helps show you're "well rounded." But it's not going to get you in, and lack of it is unlikely to keep a strong candidate out.

Good luck with it!