Well, a lot of stuff goes on at a convention. I'll start by talking about the stuff that doesn't cost extra on top of the $50 you shelled out to get in.
First of all, there will be a lot of discussion panels. Otakon for at least the past couple years has had one dedicated to Macross with quite a few people from here running it (and yours truly as the staffer in charge of keeping it running smoothly, heh). I don't know of any Macross discussion panels going on at Anime Boston this year, however, but you might find panels on other things you find interesting. Some of these panels are also Q&A sessions with voice actors or other people in the industry.
Second, there will be various entertainment venus. Game shows, music video contests, the masquerade, a dance, etc. There will also be quite a few video rooms where you can check out all sorts of anime -- some brand new and some classic.
Third, there will be lots of opportunity for people watching (look out for tons of cosplayers). People watching has always been one of my favorite things to do at a con.
Finally, we get to the dealer room. Generally, the dealers all have a pretty decent stock (although I'm not sure if it's quite enough to count as "en masse"). The prices may not always be the best, but you never know if you find hidden treasure that some dealer doesn't understand he possesses -- I've heard stories of people finding very rare art books and the like that go for a couple hundred bucks on eBay and such and managing to pick them up really cheap.
Hope this answers some of your questions!
- Lou