I've done the kids' clothes thing...there is a store here that is not consignment, but will outright buy your things. I've done the used book store for credit only. I'm a member of a survey panel but apparently I never fit their demographics because I've yet to get past the "screening" portion of the surveys. My family used to recycle cans when I was growing up for extra dough, but now we have curbside recycling, so that's not really an option (and we don't drink soda or beer, so no cans anymore).
My best way of coming up with a little extra is to do some freelance work and HIDE THE CHECK. This year, I've got a stash set aside for Christmas already. I also babysit for $$ occasionally. I faithfully submit rebates if we have them.