I came up with a challenge (I think, I couldn't find it anywhere else!) where you build a house or other lot, but you can't use any object more than once! So you can only use a wallpaper for one room at a time, one flooring for one room, you can't use different swatches of the same item, and so on! Here's the video I made of me doing the challenge, I'd love to see what more people come up with!
https://www.youtube.com/watch?v=2SvdlNugUxI