The Falls are beautiful (and the boat tours that bring you right up to them are pretty cool) but there are too many other tourists for me to really enjoy it. :P I think my favorite place to see was Fort George but I'm a history nerd and they had a musket firing demonstration. :P
Sounds like it will be a fun date indeed! I think dates based on historical places are the best kind! I always wanted to take a date on a Boston Harbor Cruise and visit George's Island in the harbor. The cruise is like a history lesson of the harbor, and George's Island is an old fort/Civil War prison that is supposedly haunted! Makes for a great day!