Meaning: extremely comfortable
Rewrite 1: as cozy as a cat on a cushion
Rewrite 2: as delighted as a dozing dog
Comment: You want to keep the flow and fun rhythm of the original, in this case using alliteration. You also want to provide that same sense of warmth and comfort.
Writing Tip: It's so easy to resort to a cliché, we sometimes slip into it without thinking. When you write a comparison or a metaphor, read your words out loud. Do they sound eerily familiar? Did the words come to you easily? It could be you're using a cliché, intentionally or not. If so, check a book on clichés or do a search on the Internet. You might be surprised at how many clichés there are and how many you aren't even aware of!