Menstrual cups. I'm straight up not having to buy pads/tampons anymore. (Aside from keeping an emergency stash stocked). Saves around 10 a month.
Handheld French press. All I need is boiled water and coffee grounds to make my own coffee anywhere. I choose this over coffee pots and keurigs at home, but it's also super convenient when traveling. (Hotel/Motel coffee makers can be super gross if you look too close.)
Buy frozen fruits veggies when they're cheaper. Freeze them yourself when they're not. There are reusable bags made for this, but using and cleaning out a decent/sturdy ziploc bag works for me.