Here are a couple of projects that took less than an hour to build using mainly materials found on the curb

Step 1: Walls and Roofing

OK I cheated a little here because almost all of the walls are made from recycled fence panels,
however each panel only takes a couple or minutes to erect, dig a hole, insert a post, tamp it down and attach a fence panel standing on it's side for an 8' wall or attach it normally with about a foot of space on the bottom for a 7 foot tall wall, more panels can be laid across the hallways to make ceilings. Extra pickets can be used to make a really rustic looking gable end

Step 2: Rustic Bar

This bar literally took 20 minutes to build but with a little more time spent on a frame it would make a great addition to a man cave.
We took 2 sets of louvered closet doors and cut them to a comfortable height for a bar (sorry we didn't use a ruler) 2 short posts were sunk in the ground and a cross piece added then the cut louvered doors were screwed in place, an old door was screwed down for the top

Step 3: Hay Roof

we wanted a different texture for one of the rooms roof so we went with hay, the ribs are commercial adjustable
shelving strips that was covered with wire fencing and a bale of hay spread across the entire area, clear plastic
keeps it from blowing away

Step 4: Cemetery Ambiance

In the cemetery we used a couple of large branches and 2 large bags of spanish moss, the moss is scattered around on the "trees" the fencing and tombstones

Step 5: Infinity Tunnel

A lot of our fence panel "hallway walls" we covered in carpet padding rescued from carpet store dumpsters at the end of this hallways is an infinity tunnel. We took a rescued patio door with mirrored film on one side and attached it to a frame of 2x4's about
2 feet back we added another mirror, filled in between the 2 with painted plastic and backed the plastic with twinkling red and orange lights. The 2 mirrors reflect each other giving the illusion of a long hallway, our frame wasn't completely square so the "hallway" seems to turn to the right as you approach it