I’ll be honest with you guys: I had no idea that decorated graduation caps were such a big thing. I know, don’t I sound like your typical mom trying to be cool by pointing out Internet trends you’ve known about for months? Cute! Anyway, back in my day (cue crickety old man voice), we all just wore our grad caps as they were. Sure, some super creative people added a cool extra tassel or their initials or something, but no one was full on painting their caps.
Teaching Idioms? I know I'm preaching to the choir when I say that idioms are more fun than a barrel of monkeys. There is a boatload of idioms at GoEnglish.com. Beyond going over the literal meaning of such phrases as "It's raining cats and dogs!" there are many other out-of-this-world things to try. They are the cat's pajamas, so give them a whirl!