(1) I think something fun would be seeing how many RNGs we could come up with within a set time period. You could possibly segregate these by type as well. ^.^
(2) Assigning people RNGs they haven't already collected is another slightly less original idea.
(3) Trying to find your seed on the first attempt? (no set time limit; long-standing)
I have a few more ideas, but I'm thinking that most of these challenges would have to be made available for over four days, if not unlimited. Making a checklist of specific accomplishments seems more reasonable for RNGing, imo.