A friend asked me how I hung the metal trays, and since it was a matter of trial and error, I'm posting it here for you guys in case you want to try it. I used the same method for these metal trays painted with chalkboard paint that we bought for the video guest book at our wedding.
Hanging them was harder than expected. I tried those stick on hooks that you put one strip on the wall and one on the object. Didn't work. I tried picture hanging wire attached to the tray with duct tape, which worked for a week or so, but then the trays eventually succumbed to gravity as the duct tape slipped up the tray.
The current solution is picture hanging wire made into two squiggles and a loop and attached with super strong glue made for metal. It seems to be working well, and they've been up for a few weeks now.