Shanghai is a massive, often overwhelming city, a swirl of noise and smog that could leave you dizzy. Instead of taking Dramamine and hiding in your hotel room, a possible day trip antidote could be Shengsi Island. Not the most spectacular plot of land in the ocean (no foreboding cliffs or coral-strewn bays), Shengsi shines because of its fresh air and ocean breezes, the handful of clean, sandy beaches, and stellar selection of local seafood. Take the ferry from Shanghai (1588 Waima Lu) and after an hour or so you'll be on the island, which is small enough to explore in a day's time.
Check out Nanchang and Jinhu Beaches for places to relax and swim during the day (both of which are also fine to camp on at night), while your search for food and accommodation will bring you to the village of Donghai Yucun, where the fishermen begin gathering in droves in September.
Note:It may be in your best interest to make your getaway during the week, because on the weekend costs will be as high as triple what you'd expect to pay in an effort to gouge tourists.