Hit up the old downtown area... On early arrivals I like to go downtown to the little grocery store right on the harbor... get some bread and fruit then sit down there and snack while watching the people.
Walk the shops (lots of them), be sure to hit the fudge place a few (2) blocks up from the harbor, the condom store (Condom Sense) place right next door for a good laugh, then head back down to the water and eat out on the deck at portland lobster co. Beers, 2 lobsters and sides for about $18, IIRC (been two summers since I was last there).
Drive out to the light house and if you have the time head up to Freeport. It's a great drive and the scenery is really nice. Plus there are plenty of little places to stop along the way to get off the beaten path.