I think a good guide is ask yourself "if this was an attractive woman I'm looking at, what would I say to her"? Usually nothing. If anything I might say something like "hi, how's your day going" or "hi, how are you".
There were a couple of Halloween outings where we (wife and I) were in a not so crowded bar and on a couple of different times different guys walked by our table and gave me the two thumbs up sign, and kept on walking. To me this meant "good for you" and maybe even "I wish I had the courage to do that". I was perfectly fine with this level of acknowledgement.
One time when I volunteered at a Pride event for the day (fully made up), a couple of drag queens were walking past us (me and a gg). One came over, gave us both a kiss on the cheek, and kept on walking. A little different because it was a drag queen, but it was a nice acknowledgement. The gg was tickled pink as well.