Many things have already been mentioned, but:
Feelies. Could be anything from a keychain to a T-shirt, really. Lemmings 2 had a keychain, Ultima had nice cloth maps, and Wishbringer had this little glow-in-the-dark plastic pebble. Even today I am willing to shell out some extra money for a game box with a feelie (I purchased The Whispered World two times and even fell for the boxed version of Terraria).
Knowing that there's a little game-related goodie in my box as soon as I provide kickstart, well, that'd be good enough for me.
Ingame-cameo sprites would be my second place, but that's only because I am HIDEOUS.
As a player/consumer
, I would probably be very exited about the opportunity to meet a developer I already know, but meeting some random guys who make an indie game would be less exiting to me (as self-proclaimed AGS indie game developer I would be thrilled, however!)