Being able to roam a map without joining a server is always useful. Thankfully you can do this in Midair, though it isn't obvious or fully supported by the developers. Follow these steps to play any map without joining a server:

1. Start Midair
2. At the menu screen, open the console (~) and type "open " without the quote. Make sure there's a space after the word open.
3. You will be shown a full list of maps within Midair to choose from. Use the arrow keys to select a map, and press enter to select it.
4. You're all set! The map will load and you're now free to roam about it.