Running.
Was supposed to be the cheapest way to get exercise. You can do it right from your front door, no gym subscriptions, no specialized equipment (some people will tell you you don't even need shoes), and it's far and away the best time-value exercise I've ever found. You can get away with like 20 minutes 3-4 times a week and be doing great.
Well, turns out I love running and I love distance running so I'm now putting up enough miles to need new shoes 2-3 times a year, a nice Garmin smart watch and heart rate monitor to track my progress, sign-ups for several long-distance races each year, shorts, socks, you get the picture.
Could I do it cheaper? Yeah. But at the end of the day it's a hobby and I like it