Very very secret rooms with some stuff you think is funny. Quake I has a Dopefish cameo in Crypt of Decay, CliffyB appears in Unreal Tournament, Doom II had several hanging Commander Keens on hooks, etc. (there is a secret in Tomb Rabbit Central Cave, although I *never* got there.)

I doubt you will have something really original. It's really hard to do a original puzzle (no, I won't say what I did once) or something other.