Mailbox flag: a piece of aluminum from an old baking pan, cut with tin snips, smoothed with a file, punched with a hammer and an old nail, then buffed with sandpaper. A few old screws to attach it to the wood that sheathes the mailbox and restrict its rotation. It mostly works, just a little prone to putting itself in the down position.
Curtains: cutting the fabric into panels and hemming them by hand. This is where it pays to have chosen fabric that you like; it takes a little time to hem multiple yards, even at two hem stitches per inch.
Baby fence for laundry room: the opening is more than five feet wide, and what I opted to do was to buy an old oak door from the Habitat for Humanity ReStore and place it on its side across the opening. The door was $35, although there were crappy old doors with layers and layers of old paint (lead paint??) for as little as $5. This particular door had a swinging mechanism at the top that I took off and was able to use to secure one end of the door to the wall. The other end is held in place by boxes for now.
Furniture reassembly: much of my furniture was inspired by the book Nomadic Furniture, and either breaks down or folds up for moving. I finally got the last piece of furniture (a kid loft) assembled. Note to self: LABEL the pieces of this loft next time, to make this process go faster.
Things on walls: I hung up a length of thin rope in the room that seems destined to be our homeschool room (despite our very informal homeschooling approach), across the wall so that I can hang up the children's artwork with clothespins. Fast, easy, and cheap.
Next up: figuring out how to actually hang the curtains, at the moments some are just draped over the rods, and the rest are held up by clothespins hiding in the background.