It might be redundant to propose marriage while inside a church, but why not! There's a number of beautiful and historic old churches in town: Arlington Street Church (right across from the Public Garden), Trinity Church in Copley Square, Old North Church...Those three and more are open to the public. I bet if you talked to somebody at one of them, they'd be into "conspiring" with you and would agree to perhaps set the stage by arranging a "private guided tour" at a reasonable hour.
Mid-March in Boston can surprise with pleasant weather, not much different (I'd suspect) than Minnesota. The huge St Patrick's Day parade here, always held on the Sunday closest to March 17th, rarely has rain or cold interfere. If you luck out with the temperature - since rain can be romantic - there's a strip of parkland along the river, called the Esplanade. As sunset approaches, walk along there until you find a bench or grassy area that allows for a sweeping view westward. Or start crossing the Longfellow Bridge (between Mass. General Hospital and Cambridge) or the Harvard Bridge (on Mass. Ave between the Back Bay and MIT.) Those are lengthy spans which also offer panoramic vistas of the city skyline and the day's end. Uh...don't try a bridge if there's a chance she could scream and knock the ring into the river!
Christopher Columbus Park is also a great suggestion for that "by the water" effect.
You realize that by posting here you're going to have to check in again on the 18th?
