#1 Unless you really need to don't drive in the central part of the city. Boston drivers don't signal otherwise they would give their strategy away. I don't know exactly what it costs now but when I lived in Boston five years ago the 'T' (the subway/tram thing) cost 85 cents in one direction. So for example you would pay to go out but the train in the in direction was free.
#2 If she likes to shop take her to Filene's Basement. Forget the upper part of the main store because it is just like any other stuffy department store. Downstair however is the home of some of the best bargins anywhere in the world. When I moved to the UK I bought a years worth of clothing before hand at Filene's for under $100.
#3 If you don't mind being laughed at go on one of the duck tours.
http://www.bostonducktours.com/ IF you do mind take a trolley tour. I've done both, they are both decent tours of the city but the duck tours are just a bit more silly.
#4 If you have a chance get out of the city and head for my faveorite area in the entire world. That would be the Cape Anne area. Salem is on the way so you can stop and do some witch related things. If you go to Salem I recommend the Witch Museum. It is a fun tour that as a child scared the absolute crap out of me but as an adult was just informative.
On Cape Anne go to a town called Rockport. That place is heaven on earth and has a number of nice places to eat. It is an artist town and is just on one of the most beautiful locations.
It is a bit early in the season to go whale watching (shame because it is amazing) but a short trip from Rockport is Gloucster. In addition to being the town where Perfect Storm was based (as it really happened there) they have some kick ass lobster places there. The most notable in my mind is down by the docks and is called Seven Seas, I think. The lobsters come in from the sea right next to it and end up on your plate. Great town, my faveorite in the whole world next to London actually.
#5 The combat zone has nothing to do with the military
#6 The reflecting pool at the Christian Science center (if it is on..might not be in march) is one of the most romantic places in Boston. That is on Hunntington ave, I think. Anyway it is right near the Prudental Center which is hard to miss.
Finally..if you pass Northeastern University wave at it for me.