For a little taste of something new, how about metaphors? A lot of the descriptions are adapted from Wikipedia.com.
To get learners thinking about (appropiate) metaphors:
(from Five-Minute Activities by Penny Ur)
1. Give them a subject and a number of possible metaphorical comparisons. For example:
a student: a flower, an artist, a climber, a hunter, a puppet, a lump of clay, a soldier, a philosopher
a family: a house, an octopus, a fire, a garden, a bed, a hand, a river, a chain
a teacher: a film director, a book, a counsellor, a police officer, a car, a manager, a signpost, an artist, a key, a walking stick
an office worker: a puppet, a pebble, a cog, an ant, a page in a book, a carpet
2. Ask them individually to choose the one they think most appropriate. Explaining their choice, they should compare answers with a partner.
A metaphor is an implicit comparison of two things, technically saying one thing IS another. If you take it literally, you might get a little confused!
e.g. "Life is theater", "Love is a journey", ""We are but a moment's sunlight, fading in the grass", "All the world's a stage...", "Life's a bitch", "The world's my oyster", "She's the apple of my eye".
A simile is an explicit comparison (with a word such as "like," "as," or "than") to another subject. "The snow was like a blanket."
Notable Similes (from Wikipedia)
"A woman without a man is like a fish without a bicycle." —Irina Dunn
"As good as gold." —Charles Dickens
"Solitude... is like Spanish moss which finally suffocates the tree it hangs on." —Anaïs Nin
"A mouth drawn in like a miser's purse." —Émile Zola
"Death lies on her, like an untimely frost." —William Shakespeare
"Death has many times invited me: it was like the salt invisible in the waves." —Pablo Neruda
"Idle as a painted ship upon a painted ocean." —Samuel Taylor Coleridge
For more advanced learners:
A mixed metaphor is one that leaps to a second comparison/identification which is inconsistent with the first.
eg. "Clinton stepped up to the plate and grabbed the bull by the horn", "That wet blanket is a loose cannon", "Strike while the iron is in the fire".
A dead metaphor is one in which the sense of a transferred image is not present.
eg. "money", so called because it was first minted at the temple of Juno Moneta. Though, to most people, "money" doesn't evoke images of the temple. People are typically unaware of the origin of words.
An absolute or paralogical metaphor (sometimes called an antimetaphor) is one in which there is no discernible point of resemblance between the idea and the image.
eg. "The couch is the autobahn of the living room."
A complex metaphor is one which mounts one identification on another.
eg. "That throws some light on the question." Throwing light is a metaphor and there is no actual light.
A compound or loose metaphor is one that catches the mind with several points of similarity.
eg. "He has the wild stag's foot." This phrase suggests grace and speed as well as daring.
Personally, I try to encourage learners to be creative with their use of language. It makes their English more interesting and eliminates cliches. Using metaphors and similes is just one way to achieve this.