Cooking is a big one - honestly, we could probably lower our grocery bill if we ate rice and beans more, but, y'know, roast duck in a peach and brandy reduction totally has it's place. ;) (And that place is the realization that serving that at a dinner party for 4 is cheaper than meeting friends at a restaurant and ordering a meal for 1, PLUS friends bring amazing wine/dessert).
Reading: either books from the library or friends, or bought on Abebooks (usually under5$/book, which I'm willing to pay for something I want to keep around. Being surrounded by books makes me happy).
Productive hobbies that involve improving things: learning to do basic wiring, or painting, or basic wood-working, or re-finishing furniture, or sewing/darning, or fixing things around the house, all create a sense of satisfaction and exand the possibilities of what your house can be for really minimal cost. Plus you get to feel super competent every time something gets done, which is always nice. PLUS you can make things for presents that save costs (um, NO we are not buying a 300$ dollhouse for our toddler - 30$ of wood and less than 10 hours will do, thanks). Highly recommending Youtube tutorials for all of these, FYI.
If you're outdoors-y at all, my SIL loves hiking - I personally don't see the appeal, but she gets lots of satisfaction for no money, so cheers.
The two hobby traps:
1) I'm getting into this things and need ALL THE GOOD GEAR!! *spend lots of money* No, no you do not. I 'got into' sewing by realizing that I could buy a Brother sewing machine online for the cost of paying someone to hem 3 pairs of pants (aka: less than 80$). That machine is holding strong 6 years and multiple projects later. Everyone telling me I needed to spend 400$ for a machine? Nope. Maybe when this one gives out, but honestly, maybe not - it's fine for my needs. Same applies to basic woodworking - garage-sale tools will totally get you started, and you can upgrade faced with actual NEED, not with "I might eventually need this/should buy the best quality now before I know that I'll use it enough for the quality to be worth it".
2) "I want to get into this and need ALL THE GEAR" followed by the same thing 3 months later about something else, leading to gear for 17 not-really-done hobbies cluttering the house and costing a lot of money and not being used. NO.