When my sister and I were little, we used to make forts by pulling the couch away from the half wall and putting blankets across the top. We made doors on the ends that you can pull up with string, similar to window blinds.
Things you need:
A sheet or blanket that you can cut holes in the side (or maybe a loosely knitted blanket/afghan that you can thread the string through)
String or yarn
A yard stick -or some other long straight stick like thing to make the top of the door frame (broom stick, straight vacuum stick/hose)
Step 1: Where's the door go?
In my case, we would pull the couch away from the half wall and drape blankets across it.
To make the door, you place a yard stick across the end to make the top of the frame, and drape a blanket over so it touches the floor.