BOS is unusually close to the city, which makes short outings more realistic than at many airports. The risk is doing too much because Boston feels right there.
What to do with bags
Do not assume public lockers. If you cannot store or check the bag, choose a simple ride-based plan or stay inside the terminal.
Leaving the airport
The Seaport, North End, waterfront, and parts of downtown can work with a focused plan. Weather and tunnel traffic should choose the mode for you.
Getting back through security
Return with your onward boarding pass and ID. Add time for terminal shuttle or walking between airport areas before security.