Acadia National Park is one of my favorite places on the planet. Before we moved to California last year, DH and I would go up there often. You have some great suggestions here already.
We usually stay in Southwest Harbor (the quiet side) rather than Bar Harbor. In Bar Harbor, we love
www.geddys.com for pizza and we also like the Atlantic Brewing Company. We usually are pretty casual on our Acadia trips, so I don't know of many "fancier" restaurants - we have been to a few, but none of them are standing out in my mind right now. There are lots of shops to poke around in in Bar Harbor.
The must see attraction is the Park itself. The park is covered with beautifully maintained carriage paths that are perfect for hiking and biking. (We would usually spend the better part of two days riding our mountain bikes all over the park.) There are many places to rent bicyles if you don't want to bring your own. They have park maps of the carriage paths available so that you can find a ride that is perfect for your skill level. It is a great way to cover lots of ground.) There are also over 100 miles of hiking trails. If you like horseback riding, there are stables nearby and you can actually ride horses on the carriage paths.
I would also spend some time driving around the Park Loop Road. The scenery at Acadia is gorgeous. The Sand beach is lovely (the beach scenes from "Cider House Rules" were filmed there) and the view from the top of Cadillac Mountain is awesome. The National Park service has a website at
www.nps.gov and they have a section on Acadia.
Make sure everyone in your family brings long pants and jackets or sweaters - it can be pretty cold up there, even in the summer, especially at night - you just never know.
On your drive up to Maine, I would also recommend stopping in Portsmouth NH - I know that there have been a couple of threads about these areas on the CB if you want to do a search.
I am sure that you will have a wonderful time. Maine is beautiful.