Blackout windows

If you have, say, a 3-year old* who has trouble sleeping when there's lightning out, you can use painter's paper and painter's tape to improvise a quick opaque window covering. The 3-foot wide paper is perfect for our windows. That only really works as a one-off. If it comes up more frequently, just get some big sheets of cheap, stiff, opaque material like paper board or poster board. Assemble a single piece exactly the right size to fit inside the window frame, and just squeeze it in. Or you can make it big enough to fit all the way around and over the window frame. Then just tack, tape, or hook in place (if you need it a lot, mount hooks in the wall).

* She didn't have this problem before 3, so don't assume you're safe.