Son bou and the one at Son Parc. Slightly biased because these are the only places that we have been to in Menorca, but they are nice.

Son Bou is gorgeous to walk along (3.5km ) around 7pm, when it is still warm , but bearable and oh so quiet. Mind the nudists though!!

If you are up for exploring walk over the rocks/cliffs at Son Parc, and you will discover a gorgeous and well hidden small beach where the wind is minimal. Not advisable with buggy though. In fact pretty impossible. About a half hour walk/trek.

There are loads of hideaway beaches like this on the island (found some, but cannot for the life of me remember where they were!!) and are a real treat after making the effort. Good guides point out a few.

One thing to point out though is that Menorca can get quite windy and on Son Bou the best place to sunbathe without being whipped by the sand is by the dunes. Again, when it was windy in Son Parc the beach there wasn't particulary good, as the water was incredibly choppy.
Both holidays were in June/July time.