i mow lawns all spring, summer and fall, so its definately not growing right now. golf is probably a no considering it is January and he lives in Wisconsin (any potentially customers would probably go to the caddys who have worked there the longest, meaning not you).
stick with shovelling driveways, cleaning gutters and selling your old stuff. Best ways in the winter to make a decent amount of cash. Also try reselling soda, candy or ice tea at your school since kids would be willing to pay a little extra if they can't buy it at school.