Massachusetts Institute of Technology
Cambridge is home to not just one, but TWO of the world's most respected institutions of higher learning. Two subway stops, or a 25-minute walk, away from Harvard University, lies Massachusetts Institute of Technology (MIT), one of the most reputable institutions anywhere in the world when it comes to science, engineering, and economics.
The domed Maclaurin Building, as seen from Killian Court.
This being primarily a technical research institution rather than a full-fledged university, MIT definitely has a geekier feel to its campus, and doesn't feel warm and cozy the way Harvard's campus does. Kendall Square and other areas in and around the MIT campus are short on shopping and entertainment options as well, compared to Harvard Square. Nevertheless, being so close to Harvard, not to mention other great institutions of higher learning such as Boston University and Tufts University nearby, MIT and its students can still draw off of the vibrant culture of those areas for a well-rounded experience.
In my previous Boston-area visits (1993 and 1994), the MIT campus was more or less the only thing I was able to see; for 1993, I was visiting MIT in hopes of applying there, and for 1994, I was visiting a student.