Normally I would probably make a comforter- using the flat sheet as one of the sides, but since the boys already have nice comforters and we have blankets bursting out of our closets- that's not an option.
This is what I ended up using one of the flat sheets for:
Project #1- Curtains
I cut the bottom 1/3 off of the sheet and set it aside. Then the remaining 2/3 I cut in half lengthwise. From there it was a simple fold-over and seam along where I cut it in half. Then I turned over the top 5 inches or so and sewed 2 seams, the first one about 3" from the fold, and the second about 5" from the fold (which was also the end that had been turned over. Thus making 2 tube like holes at the top. The first tube is what ends up being the ruffle at the top, and the second tube is where your curtain rod slides in. That was it! easy cheesy!
Taking the 1/3 of the sheet that I had set aside, I lined it with some extra denim fabric I had to make it extra thick and sturdy. Then I folded it in half to make a square and sewed it like a pillow leaving a 15" hole to turn it inside out and stuff it.( I sewed each seam 3 times because the last thing we want is the stuffing all over the house!)