A lot of people in SP might not know some locations , 'boss battles', and extras that exist, and this forum will (should) help you find them.

If you want to find them yourself, don't look down.

Locations

- Sky Park city

1: Start at the USS tiny.

2: Turn to heading 190 and continue straight until you see a small floating thing that is white, green, and black.

3: This is Sky Park City!

- Snowstone

MISSILES ARE GREATLY ENCOURAGED

1: Take off at Bandit Airport and head to the WWII fleet. (Thanks @FlyingPatriot for correcting the fleet name :P)

2: When you meet them, destroy them if you like, and turn to heading 345 - 350.

3: Keep going and you will find an icy island.

4: This is Snowstone!

- Maywar

PC IS REQUIRED

1: Start a Wright Airport, and turn to heading 113.

2: Keep going a long way, and eventually you should find a desert island.

3: This is Maywar!

Extras or 'Boss Battles'

- The Cthulhu (Miscalled the Kraken)

PC IS REQUIRED

1: Start at Bandit Airport and turn completely around from where you started.

2: Move toward the spire-like islands.

3: Look for the three biggest islands. if you connect them, two of them will form a 'gate' that is about in your direction. Move towards those and correct in their heading, from the 'gate' perspective.

4: Continue straight. You may notice small green islands, but ignore them, unless you absolutely like small islands and just have to go to them.

5: After flying for a decent amount of time, if you look carefully enough, you may see a medium-sized smooth tan island in the distance, that looks somewhat like an oval. Head toward that.

(Note: If you get scared easily, dont continue.)

6: Watch what happens...

-The Brown Pearl

(Thanks to @SledDriver 's post on this)

YOU NEED MISSILES

1: Take off from Wright Airport and turn around 180 degrees.

2: After you pass where you started, turn about 60 degrees right (heading 247, roughly).

3: Fly for a decent amount of time, and wait for a notification that looks like it was made by a pirate.

4: Look right, and get your anti-ground missiles ready, and then look for the closest and weirdest moving pirate ship. Hope you can move fast and shoot faster...

-The Pyramid of ET

PC IS REQUIRED
MISSILES ARE GREATLY ENCOURAGED

1: After finding Maywar, discover the location 'Pyramid of ET'. Spawn there.

2: Act like you want to pass through the two openings in front and behind the pyramid. Fly at a slow speed, and land inside the pyramid.

(Note: Air brakes and agile planes are recommended.)

3: I highly recommend creating a custom location, because I do not guarantee you will activate the pyramid on your first try.

4: Slowly (At roughly 20 MPH or less) act like you want to leave, and when a notification comes, quickly fly out of the pyramid. Get your anti-air missiles ready to fight the pyramid...

(Original: I put 'anti-ground missiles' :P)