I have some theories on what was in the chest. Seeing as how it's never brought up, it likely wasn't too important, so here's my theories:
A video or written message explaining how Pink created Steven. In case Steven was curious or Homeworld found out about her faked death and Steven needed to explain that he was his own person.
Something related to the Light games, might be a couple years before that continues though. That would explain why no one mentions it because the Light games are never talked about.
Nothing's in the chest. Pink just thought it looked pretty and figured that Steven would like it. Can we really put it past Pink to do so?