The challenge is simple: The words in your sentiment have to rhyme; therefore you must have at least 2 words in your sentiment. Having two words counts as long as they rhyme.
Let's see what Amy came up with:
And on the inside:
That is soooooooooo clever, Amy, I just love it!
If you don't have a sentiment stamp that rhymes, you can always type one up on the computer, use alpha stickers, or even handwrite it :)