How about adding a meta achievement to the Activities so that when you complete all the Achievements associated with that activity you are rewarded a unique skin related to the mini-game?
For instance:
Keg Brawl – Ale Barrel Backpiece
Crab Toss – Big Leather Crab Handling Gloves
Sanctum Sprint – Winged Sandals
Southsun Survival – A Cool Bow
Just my ideas, but it would be a pretty simple way of getting a few folk more interested in what can be enjoyable mini-games.
Thanks!