On day 5, it'll be evening by the time you get to Bangor, so get a hotel and stay here. I can recommend the Country Inn by the Mall, and Comfort Inn (which used to be a Hampton Inn so it's nicer than you'd expect), from personal experience, and friends have stayed at the Ramada. It's about a 90 minute drive out to Acadia/Bar Harbor from Bangor, given summer traffic, so plan accordingly. Honestly I'd stay in Bangor both nights. Hotels are expensive in BH in the summer, and even in Ellsworth it's pretty expensive (and unless it's the Ramada, Hampton Inn, or Comfort Inn, it can be pretty sketchy, too!) Remember that Boston is a good 4+ hour drive from Bangor, and you'll have to find time/place to park before the Sox game. Can't believe you are going to see them play the Yankees!! IF you don't want the hassle of driving into Boston, maybe drive to Portsmouth, NH and park at the bus station just off I-95. You can take C&J into Boston, and parking is free. Bus runs hourly, around the clock, between Portsmouth and South Station. Then you can take the subway to Fenway Park.
If you don't want to hassle with the traffic to the Cape to see Plimoth Plantation, how about Old Sturbridge Village? It's right off the Mass Pike in Sturbridge, and pretty historical. You could drive down from Portsmouth (95S to 495S to 290 through Worcester to I90/Mass Pike west... pretty easy drive, even in traffic), in about 90 minutes or so, depending on traffic. Also, it would make it a faster, easier drive to Jacob's Pillow. Regarding Six Flags, my daughter has seasonal passes to the 6 Flags in Springfield; she says they always enjoy their time there. Just don't compare it to WDW!!!
(If you decide to skip the Stephen King tour, just go see his house. He lives on West Broadway. If you are coming from I95, take the Union St. exit and head east (so left at the end of the exit if you are coming from 95S, right at the end of the exit if you are on 95N). W.Broadway will be on the right, just over the top of the hill after you pass the park with the big swimming pool on the right. The King house is a couple of blocks down, also on the right. It's red, with an amazing black wrought iron fence around it. I believe he also owns one or both houses on either side of the red one, but the red one is their house, where they stay when they are in town.)
HAVE A GREAT TRIP!!
