Mena covered most of it. I was in the same group, and it was a really relaxed, friendly kind of day. Most of us had 4th year students as our interviewers so it was easy to ask them questions about student life, research, etc. Make sure you eat lunch really fast or have a huge breakfast because you don't have a lot of time between getting your lunch (which was delicious) and when students come in to talk to you.
They said last year they accepted 60-70% of those who got interviews, so if you've gotten this far you should be in pretty good shape. I was a little bored during the Financial Aid presentation, so we did the math on the numbers for this cycle, and so far, its more like 75% percent of interviewees get accepted (but it's probably just because its early).
I had never been to Boston before, so I stayed a total of 3 days just to see the city, and I loved it! I'd recommend doing AT LEAST one day of touring, just to get a feel for the city and the people.