Guns. even though I only have 7, they're all super interesting, and if you get a gun used, it will typically be worth what you paid for it 20-30 years later which is awesome. Kinda cool you can get a 1911 pistol used in 1995 and can sell it for the same, maybe more, adjusted for inflation in 2023.
I also keep all the books I read. my vain side likes to look over them like trophies if I'm being honest.
