What I mean:
-Ability to revisit one-time locations on command
-Text that appears on screen when you find a ghost or DNA cluster (also: visible DNA clusters)
-Rock (and other NPC) dialogue
---The ability to review what your rock has said could also be potentially useful in other ways, down to and including a compilation somewhere so that people who hit "enter" too fast can see what they missed...
What I don't mean:
-Infinite repeat rewards
-Removal or undoing of anything that happened when you first completed the quest (badges, etc.)