It is easy to write a review if I can discover weaknesses in the paper.
That is because you comment on the weaknesses. You do this to show to someone who is convinced of the paper why you think the paper is weak.
For a strong paper, you would do the opposite: You highlight the strong points, in order to show to someone who is convinced that the paper is poor that it is not.
If it helps you, you can create a checklist for yourself, listing aspects that you expect to find in the paper. For instance:
- Is the contribution clear?
- Do I understand all parts of the novel idea?
- Are there any examples of how to use the novel concept?
Often, some of these questions are already provided in the invitation for the review. In any case, you can highlight the answers to these questions in your review, and thereby corroborate your suggestion to accept the paper.
In all the cases, I'm unable to give any suggestions to improve the approach or the narrative of the paper.
You do not need to do this. If you really think the paper is great, and if that impression is supported by the aforementioned checklist, there is no point in suggesting changes for the sake of providing some suggestions.
How to show the PC members that I didn't just skim through the contributions, and agreed with the authors
That is exactly one of the reasons why you should explicitly highlight which parts of the paper you liked. One way to show that you actually understood the content of the paper is by writing a short summary of the paper in your own words at the beginning of the review. Later on, when you highlight positive aspects of the paper during the further text of your review, you can refer to the various parts of the paper content that you already mentioned during your short summary. This should be sufficient to show to both the authors and the PC members that you based your positive evaluation on actual comprehension and thought, rather than a cursory impression.