On Github, there is a way to leave individual comments in the code and in addition give a review a summary.

In addition to hitting the "approve" button, I typically spell it out explicitly in this summary: "Please check my comments and see if anything makes sense to implement."

Often, I also take this opportunity to point out the "one" most valuable change in my opinion.

If the developer of the code doesn't find any of the comments to be applicable/usefuly, they can always go ahead and merge it right away.