Other than my own maps, I enjoyed exploring the secret areas in the Kitchen and Bedroom.
I also loved adding stories and subplots to maps, such as the government conspiracy in GSF Paradise, the alien mystery in San Andreas / California and the hidden world under NYC. They were inspired by other games and movies, and it was fun to see others try to uncover them; there are countless topics about them. There is also an easter egg in Old Miami / Vice City 2 that I don't think many people discovered.