The miles take longer to cover in Maine; no freeways along the coast north of Portland. Forget a Portland-Rockland drive if you're only there for one night.
The suggestion to just stay in the Old Port section of Portland is a good one. But if you want to get out of the city I'd suggest heading south on I-95 a few miles to Wells, Kennebunk or Ogunquit. Ogunquit and the Perkins Cove area are pretty charming, an hour or less from Portland, and there are all kinds of little lobster shacks and informal places to eat.