Davis sq is a cool neighborhood. Plenty to do at night, easy red line trip to Harvard, Central and Porter.
Certain neighborhoods in Brighton are good for late 20 yo's. Oak Square is quieter than allston and has easy bus access to Cambridge (rt64), Kenmote sq (rt57), Copley (503) and financial (501). It's also close to the pike and soldiers field rd/storrow drive for easy commuting around and out of the city.