On July 31, 2012, the Great Gnawnian Games location was opened.
First, you must collect Runny Cheese. To do this, you can continue hunting at your favorite location, as mice anywhere drop this cheese.
You can also hunt at the Great Gnawnian Games using standard cheese. The mice won't be worth much, but they'll be easy to catch.
Once you have a supply of Runny Cheese, go to the Great Gnawnian Games and equip it. A strong Physical or Tactical trap is recommended.
Any hunts you make with Runny Cheese at the Great Gnawnian Games will count towards the 100 Mouse Dash. The heads-up-display will track your progress. Each hunt progresses you by one meter. (Conversely, hunts not made with Runny Cheese will not count towards the 100 Mouse Dash.)
There are three new event mice to catch: the Hurdle Mouse, the Extreme Everysports Mouse, and the Trampoline Mouse.
The Hurdle Mouse appears every 10 meters, as denoted by a hurdle in the heads-up-display.
Catch at least 60 mice during the 100 mouse dash, and you'll get a King's Medallion (a collectible), some Athlete Charms, and some King's Credits.
Athlete Charms give a hefty 10 luck bonus in the Great Gnawnian Games. They can also be acquired by donating to the creators of the game (via the Donate button).