Hiking/scrambling is the cheapest/easiest to get into if you have mountains near you. The price of admission is a decent set of hiking shoes and a water bottle to start. It's also great exercise for your legs and glutes.
I also mountain bike for fun, just easy stuff on trails near my house.
Started getting into triathlons last year and that is good fun! Learning to swim properly at high altitudes isn't easy, and I think it helps me prepare for high altitude exercise in general.
Also have some river kayaks but didn't get out at all last year!