I will be in Boston in late September for a conference. Staying at the Marriott Long Wharf - will not have a car. What is there to do in the evening, within walking or short taxi ride that I should look into doing? A play? Nice Shopping?

A search on this forum will give you a wealth of stuff to do. Boston is an exceptionally compact city, so just about everything is a walk or short cab ride away (the horrible layout of streets makes distances seem farther).

Not too far from your hotel is Fanueil Hall/Quincy Market. A little hokey, but some shops to schlepp around in.

A short hop away is Newbury Street, several blocks of tony boutiques and stores. Right in the same area (Back Bay) you'll find the Shops at Prudential Center (anchor stores are Saks and Lord & Taylor). The Back Bay is full of great restaurants and stuff to do.

The Theater District is also a quick jaunt away, with lots of good restaurants as well. So you have many, many options. Have fun!

There is lots of theatre in Boston and there is also a half price ticket booth at Copley Square... Great time to be in Boston

If staying at LongWharf, the Bostix kiosk at Faniel Hall would be a better place to try for discount tix than the Copley one.
http://www.artsboston.org/bostix.cfm
Have dinner & stoll the North End ... Boston is a safe & easy city to navigate, and you wouldn't want a car.

Nice shopping is on Boyston Street and Newbury Street in the Back Bay. You could take a cab or ride the T. I would recommend walking to the North End for dinner.