If time permits, there's some really amazing stuff in New Brunswick, just across the Canadian border from Maine. St. Stephen and St. Andrews are worth visiting and are right near the border. There's also a large island (name escapes me) that you access via a highway from Maine (or a ferry from New Brunswick).

I've only made it into Maine as far as Calais (i.e. not far) but certainly will be there someday to explore the coast in detail.