pack a picnic lunch and take the ferry to the Cranberry Islands, not the national park ferry, the regular one. We did that on one trip, just walked the roads on each of the islands.
Breakfast at Jordan's (80 Cottage St, Bar Harbor, ME). I highly recommend their blueberry pancakes. We stopped there for lunch one day also, good as well. A nice dinner is Galyn's (17 Main St, Bar Harbor, ME).
On the quiet side of the island there is an old farm, we took a tour of that.
If you are there for July 4th, find out from the locals if there will be the private fireworks over the Sound. (It's been about 10 years since we saw these, I have no idea if they are still done.) If so, climb the mountain to watch them. Also, drive out onto the Bar to watch the fireworks in Bar Harbor.
We drove the hikes to either Echo Pond, Long Pond, or Seal Cove Pond I don't remember which. But there were no other people around since it is on the Quiet Side.
Get some Lobster Rolls and have a picnic on the Seawall along Seawall Road in Southwest Harbor. Our favorite place was closed when we were up there 6 years ago, as a local for a good recommendation.
Enjoy your time! We used to go every other year, it has been 6 years since we where up there last.