By "Portland" I really mean the part of the city known as the "Old Port," where all the shoppers go. I'm not a restaurant expert by any means, but the impression I've always had of Portland restaurants is that most are acceptable at least, and a few are exceptional. In any case, a morning or afternoon among your fellow tourists in the state's only "metropolis" (and it's a far, far cry from the normal definition of "metropolis") is entertaining. Portland's a beautiful city, great for people-watching, mostly blue politics-wise and, overall, friendly. There will be tons of tourists at the time of year you're planning on coming, but that's true of nearly any area in this state with a population more than about 20 people.

Greater Portland is rife with various things to do, depending on your interests. There are a bazillion and three beaches within a stone's throw, if that suits your fancy. If you're a shopper the L.L. Bean experience, about half an hour or so north of Portland in the town of Freeport, really is not to be missed, and boutique shopping in said Old Port of Portland or in the pricer locale of Kennebunk, half an hour south, is renowned. Old Orchard Beach is popular with tourists but scorned by locals, mostly because of its terrible schools, relative tackiness and preponderance of "clubs" and such drinking establishments; it's entertaining, somewhat, but definitely not a snapshot of a genuine Maine experience.
The big tourist hotspots really are the Old Port, Portland Head Light (in the town of Cape Elizabeth and well worth a visit for the view alone), any of the beaches, and whole towns--Kennebunk/Kennebunkport, Saco/Old Orchard Beach, etc. etc. etc. The problem with recommending places in southern Maine is that it's so diverse, and things change from town to town. I guess if I knew what you were into, I could be more helpful!