Traveling to Maine at that time of year varies by day. Friday is the worst as everyone is coming to Maine for the weekend. Once you get past Boston, which can be slow going, you could get stuck in a long backup at the Turnpike tollplaza. This unfortunately is the only way to get in if you are coming from the south. It gets worse later in the day. Other days of the week are not nearly as bad. Your 12 hour estimate will depend on day and time of travel.
I could give you a list of things to do and see but it depends on how much you want to deviate from your time schedule.
Some include:
Stonewall Kitchens headquarters and cafe in York
Numerous beaches in southern Maine
LL Beans in Freeport, open 24/7 365 days
Too many restaurants to list in Portland
Many, many beer brewers
When you get your tavel route squared away PM me and I will give you many more options.
My best tip is not to order lobster, lobster rolls and fried clams in a restaurant. They are cheaper and better at the roadside shacks. Many mom and pop stores are known locally for their lobster rolls. Google Red's in Wiscasset as an example.