Look at Lowell. You can probably live there, walk to work, party there at night and live without a car. Good transit and even commuter rail for occasional trips to Boston. If you have a car, there is nice nature to the North and the ocean is a 50 minute drive. There's quite a bit of 'urban nature' right in the city with extensive walking trails along canals and rivers. Most of the city is quite safe most of the time.