The first thing I look for is glass that would be good for 'make your own' candles...a fun booth at the faire where you layer colored wax flakes like you would do sand art...but it has a wick and can be burned if you want to.
One yard sale I even found some of the flake wax and a bunch of votive molds that somebody had bought and then decided making candles was too much trouble. My daughter works for a candlemaker so the votive cups came in handy for her.
Then I look for fabric, yarn, notions, sci-fi books, things that would work for costuming for faire (Renaissance Festival), kids clothes, and anything else that catches my eye...lol!