I fly up there from kMHT fairly regularly. Most of my trips include purchasing fuel (~8 gallons) and taking a crew car to dinner. If you take full tanks or more than their requirement (10 gallons?) they will let you have the crew car for a reasonable amount of time at Northeast Air. If you plan to be there for more than a few hours you may want to call both Northeast Air and Mac Air and figure out what the parking situation is.
Study the Harbor Visual 29 approach. Its really neat and 29 is the active runway surprisingly often. If 29 is the active runaway, it is totally worth it to request it from the tower. Ask your passenger to take photos and video out her window on the approach.
B19 is fairly far away and will take more planning, but may be worth it depending on the parking fee.
I have had enjoyable experiences at David's restaurant, DiMillo's on the Water, Gritty McDuff's Portland Brew Pub and Portland Lobster Company.
We usually Enjoy dessert at either Captain Sam's Ice cream or Beal's Old Fashioned Ice Cream
I would avoid Solo Italiano. They were ludicrously expensive and that cost didn't really add much to the experience.
There are some cool ferry tours of the harbor area that leave from the old port district near DiMillo's as well.
Have fun!