There are currently four different adventures in MilMo.
Players can click the telescope icon at the top right corner of the screen, or press the M key. Then click the compass at the bottom left corner of the map and select an adventure.
Spooky Island of Mice & Maniax can be unlocked by meeting Bryce in Mushroom Forest, furthermore, to unlock Air World you need to collect 4 Sun Token pieces and go to the top tower at Cannon Island. These tokens are found anywhere on the islands.