The Chinese military pretty much have unlimited access to the main airports of China and take advantage of that.  What this means is civil aviation takes a back seat and more often than not, your flight is going to be delayed either coming or going, or both.  They can sit you on a plane for hours and hours and it can make for some pretty horrific scenes on airplanes.  Shanghai isn’t too bad for pollution but reports of people flying all the way to Shanghai or Beijing then having to return are not uncommon, so take that into consideration and prepare for at least your flight to be delayed and it’ll lessen the announcement when you get it.

There’s shitloads of flights into Shanghai from Hong Kong and most other major cities, bearing in mind that if you take one of the cheaper Chinese airlines, you could be stuck on a plane full of uneducated, first time or few time Chinese flyers for hours on end.

Shanghai is huge, I think you could easily spend a week here and still have things to do, but 3-4 days is probably enough to see most of the best things of the city.

Head to Suzhou and the water towns while you’re there, they’re a nice day trip and somewhat less crowded (unless you strike a Chinese holiday).

Unless you want to be trampled to death, avoid Shanghai Disneyland.  Now I haven’t been and have absolutely no desire to go, but I’ve heard that’s a circus.  Unless you have visiting every Disneyland on earth on your bucket list or are particularly fond of Mickey Mouse, give this one a miss.