A ride on an open-topped bus is obviously a good option only on a nice day but that's the beauty of it - you don't need advanced tickets or a plan. You can just decide last-minute if you want to go and see the city from a level two vantage point.
There are several open-top double-decker bus routes operating in Puxi and Pudong.
This open-top bus runs from the Bund to Xujiahui with lots of great stops in between including the Yu Garden
A good place to get on is on Huai Hai Middle Road between Shaanxi Nan Road and Xiang Yang Road (you'll see the bus stops on the north side of Huai Hai). The cost per person is 2rmb.
The terminus is the Xujiahui Bus Station where many passengers will get off, but you don't have to; just hold your ground and the fare collector will come and find you for another 2rmb.
This bus runs a good portion of Huai Hai Road. You'll ride through much of the city on this bus but you won't see many side streets.
Where to Sit:
Strangely, I've seen the open-top buses running in the rain, but obviously, you want to take them on a nice day to enjoy the open-air view from the top. After you board, head for the stairs. You'll be wrangled by a fare-taker, give her your 2rmb and head up. Some of the buses have tables in the back so you may want to grab one of those if you came armed with snacks.
Getting On and Off:
I advise riding this bus for the pleasure of the ride, not necessarily to get from point A to B. This way you won't be upset if you miss a stop and you can ride as long as you like. Take a map along to follow the route.