We were there for 5 days also, which is more than enough time. Spend a day walking thru Bar Harbor. Lots of shops, reatraunts. We went on a whale watch boat tour, but it was a waste. 300 people on a ship, all trying to see the same whale hump or tail. Saw 2 in 4 hours.
Spent a day seeing NE and SE Harbor, 2 small villages worth the drive but won't take you but an hour or two to visit each.
Spent a day driving thru the park itself. Nice drive, lots of scenery and views.
5 days is plenty of time and you'll probably have a day to kick back and do nothing but r & r.
We stayed at the Oceanside KOA as we had a bunch of bonus points to use, but other nice parks on the island.