How to write a review for a strong paper? I was asked by Program Committee members to be a sub-reviewer for some papers related to my research. The PC members are big shots and I really want to impress them.

It is easy to write a review if I can discover weaknesses in the paper. However, if the paper is strong or I am unable to find any weaknesses, I feel my review is useless. 

Unlike the flaws, the authors tend to repeat and highlight their contributions. So my review is just rephrasing the contributions and how I am impressed by them.

If I'm unable to give any suggestions to improve the approach or the narrative of the paper, what should I do to write a good review? How to show the PC members that I didn't just skim through the contributions, and agreed with the authors (strong papers are often written by well-known researchers)?
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.