Visiting Acadia in October is a great time. You will miss the crowds, but as that is Columbus day weekend you might experience some significant traffic jams on the way up. Our weather at that time is totally unpredictable, so come prepared for just about anything, so much to do even foul weather can't spoil it.
What route are you taking through Maine? If you take the normal Route 1 trip there will be lots to see on the way, some require a little detour but will be worth it.
Freeport is probably the first stop for many if you want to shop and visit LL Bean. Rockland is the next stop you might want to make with lots of art galleries and a great museum, Camden is just up the road with lots of nice shops, parking can be an issue, however, Camden Hills State Park is there and a good place to spend the night. Further up route 1 is Searsport with a another great museum. Side trips after Searsport would involve going down the various peninsulas to Blue Hill, Castine, Brooklyn, all relatively short trips off the main drag. I think as previously said the national park bus, Island Explore, stops running on Columbus day which is too bad as it is a great way of getting around.
My rant doesn't even include Acadia. if I we can provide any other information- please ask.
Bob and Michele