Actually there's 2, transferable full autos is the other one.
Like Tim said I wasn't joking, I never really considered it until a fellow board member mentioned it to me just this last weekend. I've started to do some digging and it's amazing how much appreciation is there, and how liquid it can be once you own it/atf forms clear.
Land is also one that I would be keyed into, but just owning land doesn't do you much good unless you want to live or recreate on it, or it holds some other intrinsic value. Ie if it's not for personal use long term, you need to do your research wisely and carefully. It can also be hard to move if you need quick cash. Yeah it can be leveraged but then you're paying someone else interest on something you already owned.