Landscapes. If a game has beautiful landscapes (and beautiful graphics in general), I get weak. If someone made a game called "exotic landscape explorer", I'd buy it, even if it was nothing other than walking.
Also, strange creatures. Love them. Biologist in real life, and it shows when I'm gaming. "Man, how does that critter work" is a question that sometimes leads to me staring at some small thing at the side of the road for fifteen minutes in a game. If I have someone watching or playing multiplayer, they usually start throwing things at me by that point.
Closest so far, I'd say, are Morrowind (preferably modded up, though not necessary, the base game is already more beautiful, if not as shiny as most newer games) and the Myst games.