Bench raised bed made of railway sleepers. This would be great for a small veggie garden. (But not railway sleepers!
Simple DIY Round Stone Firepit. One of the simplest ways is to make a fire pit is using bricks. You can place them to make whatever shape you like, and then fill up the inside with stones. Don't forget to place a barbeque grill on top, and of course, make sure your grill is big enough to not fall inside.
Cool evenings around a fire? All of your friends will say 'count us in'--dress up the area with a wide array of plants in window boxes that help define the space, in addition to low laying landscape beds with lots of perennial color.