A lot of good ideas here already. I would recommend a model that is already showing considerable collector interest and is only going to get more interest - expensive - in the future. Say a mint P&R 629, or a nice 29-2, or a 29 Classic or Classic DX with box and tools, maybe a nice 696. Maybe some of the limited/low production special models like a 3 inch 29 or 629, the 3 inch blue and nickel Model 24, 27 or 29's or other Lou Horton models. Guns that you can still find but are starting to be higher value than when sold new. Put away mint, complete, near unfired pieces and watch them go up.

Bob