How to write a review for a strong paper?
The key principle is this: just as with a negative review, you need to demonstrate that you have read the paper thoroughly and are making your judgement for sound scientific reasons.  

If you like a paper, you should be able to say why you like it.  Beyond the paraphrased summary of goals and contributions (which should begin any review), you can spend a sentence or two saying what aspects of the work or its presentation you found particularly compelling or particularly elegant, and why.

It's also a rare paper that doesn't have any minor errors or needs for improvement, and you can also point out those, prefaced with something like, "The only things I see needing improvement are a few minor points of copyediting and clarification."

In the end, your review might be quite short, and that's fine.