I would say Old Orchard Beach is not nice, it's pretty touristy (I guess all coastal towns are in the summer, though?) and it has a big boardwalk with rides and arcade games and things. Â There's one long beach that I know of that is extremely crowded. Â I haven't explored that area much though (so I may have missed the nice parts) but *everyone* goes there.
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I would recommend someplace a little further up, like the Camden/Rockland/Rockport area.  Still touristy (especially Camden) but there are a lot of walking trails with the Camden Hills State Park there, the breakwater/lighthouse and playground in Rockland, lots of small local beaches, several lakes, a transportation museum in Owl's Head which is very close, day sailing trips, etc.  Oh, you could also take the ferry over to North Haven or Vinalhaven which are big local islands. Rockland and Camden have nice little downtown shopping areas.  I actually grew up inland but we spent a lot of time on that area of the coast - I can't really speak for other areas.
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There's also Bar Harbor up near Acadia National Park, I don't know much about the area around the park but it is very beautiful there.