There are some fantastic onomatopoeia resources out there but sometimes it can be hard to choose from such a wide range. A colleague pointed me in the direction of "On The Ning Nang Nong" poem by Spike Milligan. I don't know how I hadn't heard this poem before. Even better, my colleague led me to The Children's Poetry Archive where there is an audio clip of Spike Milligan reading the poem with such amazing timing and expression. Guaranteed to be loved by children of any age and adults too!
Better still, the wonderful people at Scholastic have created this worksheet to help the children to create their own verse to add to "On the Ning Nang Nong." You can download it for free from their website. (There's a slight mistake on the second last line of this worksheet. It should read "On the Nong Ning Nang.")
You could also have the children read the poem in groups using percussion instruments to add effect.
As for words that look like their meaning, I've found lots of examples to share with the children.
I gave the children a list of words such as tall, big, small, cold, hot, wobbly. They then needed to use an online thesaurus to search for synonyms of these words and illustrate them like the above examples.