I think Davis Square is fun and has a lot going on... That's where I'd go, out of the places you mentioned. It's very safe, hip, sorta granola in some ways (as a pp mentioned), and is the location of my favorite Indian restaurant as well as my favorite cafe.
However! Much cheaper, a little more suburban (though still with every sort of restaurant possible, several parks, a bikepath, a movie theater, and much more) is Arlington. Not on the T (probably has something to do with the lower rent average), but with buses straight into Harvard Square. It's quite nice.