Since I've already made my thoughts on this matter known, I'll try to refrain from beating a dead horse, but I can't help but reiterate some of my key points:

-you need to get back to the airport no matter what
-spending an unbroken 10 hour stretch on the town strikes me as exhausting
-Summers in SH are oppressively hot and humid; any sightseeing at all will surely warrant a shower or even a swim (the Portman pool isn't bad, by the way)
-if you stay in a central location, you might not need to spend too much time in taxis
-the taxi rides themselves provide welcome breaks from the oppressive heat... learn the phrase, "da kongtiao ba!"