On Little States, in "Florida" near the rocket launch pad, there is a hangar that can be entered most easily with the Rocket Belt. When you enter it and then turn around, you will be warped to the hangar in "Seattle".
On Little States, near the rocket launch pad in "Florida", there is a small gas station. If you land any vehicle that needs fuel to run, your fuel tank will be automatically refilled.
On Little States, in "California", you can find a building with a small tunnel at its base that can most easily be accessed using the Rocket Belt. Going through the tunnel, you can find a Nintendo 64 logo on one wall, and coming out the other side will reveal that you have been warped to "New York" on the map.
There is a cave under the castle on Holiday Island that will transform the time from day to night. This can be most easily accessed using the Rocket Belt.
If you use the Gyrocopter to fly into the mountains, you can find Mount Rushmore with Mario's face replacing George Washington. If you shoot Mario's face a few times, it will turn into Wario's face.