Trailmakers

Select Treasure Island from the main menu. Once loaded, spawn a plane and fly to the highest floating island in the middle of the map above the peak of the mountain. The island will have a monolith-type slab standing on it and a billboard. Land or crash the plane into that floating island and approach the monolith. Achievement should pop when you get close.

Shout out to Retrochief for the guides! I made a video to help if needed

Achievement Walkthrough:
Monument: 1:20
Tree: 2:00
Hoop: 3:15
Top of Danger Zone: 3:50