Do "odd" jobs, like mowing lawns, flower bedding, garden/weeding for neighbors or elderly; do errands for them (or single moms). Put up flyers around the neighborhood, hand them out, etc (keep a few in your pocket so you're ready).
(Yes, that would be dependent on how close the store(s) are, etc. But, a bike is a better investment than a car is, for sure. A bike, or a big red wagon)
Walk dogs (for a fee).
Learn a craft--sewing, crochet, etc. This is more of a long-term skill (as in, it takes a while to get good at it), but it could bring in some money.
Pick up litter around the neighborhood (for a fee)
Hit up garage sales, estate sales, etc., keep an eye out for things that can be sold for much more than it's bought (A neighbor of mine was walking thru an alley one day, and saw a fuel injector assembly for a Mustang lying next to a dumpster. Brought it back, sold it on EBay for $80 + postage)
Umm...anyway, there's a few ideas to start or initiate other ideas. Good luck!