Remember that MIT classrooms are a scheduling possibility, especially during the summer and during academic breaks.
The good news: these rooms are usually spacious and don't cost a cent to use.
The bad news: many classroom set-ups are not conducive to meetings or certain other kinds of events, and no food is allowed.
All classrooms are booked through the Registrar's Office. They have a room search form which helps you find a room that meets your criteria. Call them at x3-4788 or use their online classroom reservation form.
For unofficial, impromptu use of classrooms, there's now a handy app that helps you find classrooms not currently scheduled: MIT QuickRoom app.