Salem is about an hour by commuter rail.
Don't go to Salem, there's plenty right here in Bean town!!!!
If you like the club scene, then Landsdowne Street is the place to be. If you're a little more mature (no offence club hoppers of the world), then the MFA is fabulous. And don't worry about traffic -- just take the T. The Science Museum is also fairly good, if you like that.
If you can get an expensed dinner, the view from the Top of the Hub restaurant (top of the Prudential Center) is fabulous -- aside from the cookie desert, the food is dreadful -- and terribly expensive.
If you like burgers, The Pour House on Boylston, across from the Hynes Convention Center (and a little bit west), is a fabulous place for burgers. Saturday nights is half priced burger night -- you can get an excellent burger, as you like it, great fries, and a drink, for less than MacDonalds. I went there all the time before I became a vegetarian.
I'm not sure if the looking pool is full, but the Church of Scientology on Massachusetts Avenue is very pretty, as is the Boston Commons. If you can afford to waste some money, the Children's Museum and the Computer Museum are right next to each other, along with a giant milk bottle -- don't ask. While as an adult you probably wouldn't want to spend too much time there, it's fun to just wander through.
Also, check the Boston Globe to see if there's anything scheduled for that weekend --
http://www.boston.com .
If you can get tickets to Blue Man Group, do so. The show is wonderful. Although be careful that whatever you wear is washable -- depending on where you sit, it could get messy.
If you have any desire to go shopping, the expensive shops are all along Newbury Street. For more "normal" shopping, take the Green Line outbound to Lechemere. Two blocks from there is the Cambridge Side Galleria -- one of the best malls I have ever shopped in. And it has a Cheesecake Factory in it. Good grub.
But for a more Boston experience... try Legal Seafood. Expensive, but I've heard it's very good. (Again, that vegetarian thing!) Also, the limeade is reportedly wonderful.
Okay, that's all of the top of my head. If you have any specific interests, feel free to PM me and ask about some random topic.
Enjoy your visit!
(And dress WARMLY -- it's freakin' freezin!)