I do gardening (easier for me than most as a landowner, but there are also community gardens where I live), make my own bread, cooking, and yes, walking.
Birdwatching is good and doesn’t require buying much.
My kids lately have been into puzzles and karaoke.
There’s a lot of fitness stuff you can do without buying stuff. Yoga, bodyweight exercises, even the partner acrobatics are fun and certainly a challenge.
Sex, lol. Not a hobby per se, but so good for a relationship and a great activity to do together.