Similes are another easy craft to teach your young students. As with onomatopoeia, you may find that the first few attempts at using it contain similes in every sentence, but after they have had more experience with them, students will learn how to use them more effectively. A great book for introducing similes is Quick as a Cricket by Audrey and Don Wood. Once again you can start a chart with examples of similes on it. Provide sticky notes so that students can add similes to the chart that they in their reading. Periodically read over the new ones and write them on the chart.
Some great follow up books for teaching similes are The Seashore Book by Charlotte Zolotow and Owl Moon by Jane Yolen. Both of these books contain lots of descriptive, poetic language and interesting similes.
A third easy way that students can add interest to their writing is by using strong verbs or actions words. I usually start this lesson by talking about the word walked. I say "The girl walked out of the room." Then I ask students if they can tell me anything about how the girl was feeling or why she walked out of the room. Then I change it to "The girl stomped out of the room." Now they tell me that she is mad or angry. What if she tiptoed out of the room? I continue to change the verb (skipped, stumbled, hurried) and we talk about how changing that one word can change the meaning of the sentence and can really show you what it happening. Then we talk about how using strong words makes writing better and more interesting. Kitten's First Full Moon by Kevin Henkes is good for this lesson. You can talk about how Kitten raced, sprang, and tumbled.
These elaboration mini lessons for writing will really help your students start to add details and interest to their writing. Look here for a lesson on how to encourage your students to try out what you teach in your mini lessons.