I would walk around Boston. It's not huge and sprawling, and there's a lot to see and do that you might miss if not on foot. Get some chowdah!
Walk around Vegas, too, but BRING WATER! Srsly. You'll pay like 3 bucks for bottled water if you forget to fill your own from the tap, and it will be hot and dry. My skin and hair loved it there, though! Don't take the bus in Vegas - monorails and trams, sure, but the regular bus took us about an hour and a half to go maybe 5km. I would have gotten out and walked but was with a big group.