Beijing is and will probably still be oppressively hot...not much fun for exploring tbh. If want to just lounge in a nice hotel and enjoy good food, HK will be much nicer, although the weather will be even worse. For a beach, I have to admit I've never been to Sanya, but that is mostly intentional: can't see the appeal. I would recommend perhaps Thailand or Vietnam as preferable alternatives, which will only be a little more distant than Sanya to SH.
tb