Paper cave: I think ceilings are underused for interior decorations. Would love to try some more colorful paper or plastic cones on our ceiling. At least in a small section. It would also be a great way to add too a "man cave".
DIY origami bipyramid tutorial -- this took me a lot of trial and error to get it right. The instructions are decent, but I simply couldn't get it to roll into the pyramid for a long time. But once you do it once, it gets much easier each time.