No interest in paying gym fees, no interest in buying expensive equipment. What are the options?
The answer that works best so far for me?
Jump rope! That works your legs. And man, is that aerobic. I often do that before a cold ride to get myself good and warm before leaving the house. And helps to keep my coordination tuned. I bought that for less than $10 at walmart. You could DIY for cheaper still.
I integrate it with a regimen of other less aerobic exercise so I periodically go do a set of 100 reps on the jump rope while making my circuit among other stuff like push ups, sit ups, squats, etc.