I started wall climbing and bouldering with my GF. The time investment is that we go there two mornings a week for ca 2 hours, and some weekends for longer sessions. Living reasonably close to the gym makes it rather low effort.

I like it as it combines multiple skills such physical strength, climbing specific ability, balance, problem solving endurance etc. It feels like there's always new things to develop. Indoor climbing has the advantage of being less weather and season dependent than outdoor sports. Bouldering has the nice property of not requiring someone to do it with and you don't need to book anything in advance or follow fixed schedules.