I use a schwinn airdyne excerise bike. It's still a machine, but it's somewhat affordable - pretty easy to find a lightly used one for ~$300 on ebay. On the advice of a trainer I am currently doing intervals of 15 seconds flat-out and 45 seconds recovery for 10 minutes plus a couple of warm-up warm-down minutes. Interval training appears to be unusually effective but nobody seems to fully understand the mechanism, just the results. For a while they were calling it "oxygen debt" but that's proved to not be the case. Whatever is going on, it seems like a way to get ~45 minutes worth of cardio in ~15 minutes worth of time.