Today that secret payload was revealed!
From SpaceX’s press release:
SpaceX Reveal’s Dragon’s “Secret” Payload
Before the successful launch, voyage, and recovery of SpaceX’s Dragon Spacecraft, the first time in history a commercial company has recovered a spacecraft from orbit, reporters were buzzing with news of a “secret” payload, stowed on board.
It was a payload so secret, SpaceXers made it Top Secret (think Val Kilmer 1984, not official US Government).
So what was inside the mystery package? Their tribute to Monty Python.
A wheel of cheese.