Was just there this weekend. Despite having lived in Plymouth for 8 years, never did the duck boat tour until this weekend. I second that. Definitely enjoyable for 90 minutes or so.
We hit a few memorable places to eat - Prezza in the North End, Stoked Pizza in Brookline, Flour Bakery and Mike's City Diner in Back Bay. All excellent and highly recommended. Prezza is off one of the side streets off Hanover. Not sure about the whole "sit down" for a cannoli idea. Both Mikes and Modern Pastry were packed with lines down the block on Sunday night at 8. Maybe you could get in there during the week, but if you're going on the weekend, that seems like more trouble than it's worth.
We caught Dead and Company at Fenway and I know there are an additional 20K people there for a concert and probably a ton more that just hung around outside that didn't have tickets, but good lord, that was a mob scene trying to get out of there. If it's like that for a Baseball game, I wouldn't go within a mile of the place.