How to replay the Gravitron

Collecting trinkets unlocks songs from the jukebox.

Collecting all trinkets unlocks the trophy room, where you can view the achievements you unlocked:

• Game completion achievements:
  • GAME COMPLETE
  • FLIP MODE COMPLETE
  • MASTER OF THE UNIVERSE (no deaths mode)
    
    
• Time trial achievements: (win the respective time trial with no deaths and all trinkets)
  • SPACE STATION 1 MASTERED
  • LABORATORY MASTERED
  • THE TOWER MASTERED
  • SPACE STATION 2 MASTERED
  • WARP ZONE MASTERED
  • FINAL LEVEL MASTERED
    
    
• Number of deaths achievements:
  • Win with less than 500 deaths
  • Win with less than 250 deaths
  • Win with less than 100 deaths
  • Win with less than 50 deaths
    
    
• Super Graviton achievements:
  • Last 5 seconds on the Super Gravitron
  • Last 10 seconds on the Super Gravitron
  • Last 15 seconds on the Super Gravitron
  • Last 20 seconds on the Super Gravitron
  • Last 30 seconds on the Super Gravitron
  • Last 1 minute on the Super Gravitron

Additionally, access to a Super Graviton is unlocked. It is, however, harder than the one found through a normal playthrough. Watch a video of the Super Graviton in action.

After spoiling myself and watching a few videos on Youtube I can say that yes, that's all that can be explored in the demo version.

What the demo fails to explain is that actually you start a "level" by reaching its starting sector in free roam through the game's world. That is, the full version does not yank you to a level selection screen after rescuing the first crew member. The opening in the pink area allows for further exploration.