The area around Northeastern is not just filled with undergraduates. There are a lot of medical students and professionals in the area as it is close to the Longwood Medical and Academic Area. Personally, I would recommend Somerville, which is right next to Cambridge. If you get a monthly T-pass, it is not that bad of a commute to the Northeastern area via the Red/Green lines. Somerville is a bit more inexpensive than many of the other areas in the Boston area and has quite a few good restaurants, cafes, etc...