You know I like little games like puzzles...Really makes you think and it's even more fun to do this with a friend. I think Portals although not adventure will make a nice little game...Right now a guy is re-creating it for Gmod and I really look forward to this.
You can view it here if you're interested:
http://forums.facepunchstudios.com/showthread.php?t=181310
It gets better as you scroll down and you'll find this:
http://www.youtube.com/watch?v=x6IYfW-WXAU
http://www.youtube.com/watch?v=x6IYfW-WXAU
As I said i like little mind games...Not the boring chess or cludo mind...But the puzzles that you actually have to involve yourself in aka move objects and stuff.