Three keys unlock the principal's office, Mr. Burn's fireplace and Krusty's Office. The key to the principal's office can be found behind the bar in Moe's Tavern. It is in the rightmost box by the trashcan. The key that unlocks Mr. Burn's fireplace is on the Stonecutter's podium (in a box). Krusty's office is opened with the key found in the middle drawer of the mayor's desk, hidden in the upper left-hand corner.
Donut Toss
To play the donut toss game, go to Homer's bedroom and grab the note from the rightmost drawer of his desk. Then run to the power plant ( Sector 7G) and pull the lever on the wall. Yank the rope that falls and snag the donuts!
Go Anywhere
To go anywhere instantly, click on the map. Hold Shift, S, and V then click twice on a location and let go. The fastest way to get all 75 cards is to walk back and forth between Mr.Burns' office and the Reactor Core and collect the same cards.
How to get to Bart's Treehouse
This was kinda hard for me to find and I thought it might be for others too. To get to Bart's Treehouse go into the TV room. Look in the bottom left hand corner and the is a tiny bit of light coming through. click on the light and you will appear in the Simpson's Backyard and Bart's Treehouse in clear view. You get to chuck water balloons too.
All Keys ...