Years ago I bought an insulated coffee cup and water bottle (klean kanteen) that get used every week, especially if I travel. When I get iced coffee and I can walk around sipping it and there's still ice floating around hours later. Plus I can avoid single-use plastic cups if the coffee shop fills personal ones (some even give a discount!). The water bottle has saved me from having to buy overpriced bottled water dozens of times over.

Another one is my bike. I bought a used Marin Muirwoods commuter bike from a local co-op and it's saved from having to drive to any place less than a few miles away, like the gym. Less gas burned and wear & tear on my car. Plus it's fun cruising around and is great exercise.