Take a look at the T map and Google a few destinations and the times. Coffee shops are everywhere and so are groceries. The north end of Boston is more dense with good restaurants and more central to all of the T lines. Being along or close to one of the lines will make trips easier. If you want to spend more time in Boston, note that to get to the green line you have to transfer if you’re closer to the red line in Cambridge. The green line recently expanded more north so make sure you have an updated map.

If you look at crime statistics, this is one of the safest cities to live in. Lots of people walk around at night safely but still check the crime maps for your specific area.