There are so many idioms in the English language that we don't even think about. Some are funny and some not so much, but they make our language very colorful.
There's a list of commonly used idioms and their explanations here: http://www.usingenglish.com/reference/idioms/
Here's a cool activity where students can make their own books full of funny idioms. http://getinthefold.blogspot.de/search/label/idioms
You can also print out these funny illustrated idioms from Kaplan International. I've printed them and scanned them into a format that is printer friendly. It's also possible to have them blown up into posters. https://drive.google.com/folderview?id=0B91PszKtVKWkTE45Q20wNU5adHc&usp=sharing
There's also a site that has an alphabetical list of idioms in English and their meanings. http://www.idiomsite.com/ If you want a list that's catagorized then look here http://www.learn-english-today.com/idioms/idioms_proverbs.html I really like that they are separated out by theme. So if you want you could do a whole month's worth of lessons on idioms separated into topics.
Another idea is to have students go online, read the idiom used in a sentence and then guess the meaning. You could also make up sentences and have them put in the right idiom as a class. If you used a PowerPoint or projector it would go a lot faster. But an alternative idea is to have students make up their own sentences. Here's the link http://www.manythings.org/slang/slang7.html.
I've had a lot of fun getting students to act out the idiom on the card that they chose and having the other students guess what it is. The student acting can say one word, but it can't be part of the idiom. Try this in your classes and see what happens.
For those of you who teach in a German speaking country, here's a list of English idioms and their German translations. http://german.about.com/library/blidioms_eng01.htm