You can do the Freedom Trail on your own, or get a tour, but that would be the place to start. Once you finish that which covers a lot, including the Constitution, there is so much more and/or more depth to study. I would disagree that you can see enough in a day as I've been here for almost 40 years and I am making discoveries in Boston (and surrounds)...but you can always come back! Best hotels for a one day visit...well there are many downtown so it depends on your budget. The Parker House is right near the Common. The Four Seasons Hotel is across from the Public Garden (my favorite Park). The Bostonian is next to Faneuil Hall Market Place. Rowes Wharf Hotel is near the market place, but on the harbor as is the Marriot. Hotels in the Back Bay would be fine as well. The financial district gets a little empty when walking back to the hotel late at night so I wouldn't stay in that area.
As others have said, Boston is a walkable city, and if you are tired, just hop on the subway. You wouldn't want a car.