Is there a local events calendar you can raid for something to do? Depending on where you live, there might be art shows, talks at bookstores or libraries, and stuff like that to keep you occupied. Maybe you could offer to help your sister out with some ordinary household stuff? That way you wouldn't feel like you're taking up her time, at least.

If you can't get a ride for a yoga session (which sounds like a great idea!) maybe there's a video yoga workout you could borrow from the library or find online. My favorite "workout" videos are Bellydance Fitness for Beginners - serious workout but great moves (bf would probably love it!)

I tend to use up spare time by making stuff. All kinds of stuff, though admittedly more when I'm hypomanic. I knit, scrapbook (not with all the fancy stuff, but with really scrappy stuff), cook, bake, take photos, sew, etc. One of my current low-key projects is "Wreck This Journal" which is a lot of fun. Is there anything you like to make?