Personally, the absolute essentials for Shanghai would be The Bund, a wander around the former French Concession and a few drinks/dinners at some of the restored hotels or bars on the Bund. Plus of course maybe a trip on the Maglev train to or from the airport at 431km/h!

All of those are nice and easy and relaxing without trying to cram too much in. A day trip out would be a nice extra, but not essential.

If you want there are also a range of tours, from personalised 'luxury' to more adventurous if you want someone to take care of everything. They can be tailored to your specific needs (such as antiques, or architecture, or history etc)

For more of a taste of Shanghai, consider staying at somewhere like the Astor House Hotel. (link here, don't worry about the different name in the link! http://www.pujianghotel.com/). That will steep you in history from the moment you check-in! It's not 5 star, but very well priced and a terrific experience. (shop around for the best price, the rates listed on the hotel's site are very expensive)

Drinks - head to the Long Bar (1920's restored) at the Waldorf Astoria for live singing/jazz, and visit the Peace Hotel. Dinner at maybe somewhere like Lost Heaven (Bund branch, bookings essential).

I would therefore spend the day of arrival, a full day after, and leave for hong kong some time in the early to mid afternoon on the third day for Hong Kong.

Hong kong - so much to do without even trying... the Peak, star ferry, riding 'ding ding' (the trams), shopping, food, a bus to Stanley markets..and you haven't even left the Island yet (well, except for the ferry ride!)