Stay in Ellsworth and make the 20 minute drive to the park. Hotels are 1/2 the cost. The park in ALWAYS crowded during summer between July 4th and Sept. Before the 4th is a bit less so.
Parking in Bar Harbor in horrible. Most of the LEO's on the island are young and like to write citations for even the smallest infraction. I actually got a ticket for going 32 in a 25.
Coolish mornings and evenings. Temp only hit 90 one day last summer. It doesn't really get what we think of as hot here.
Lots of great vistas to see that aren't in the Park proper. Highway 1 from Brunswick up to Machias is a great day trip. You'll most likely get caught behind a few pirate parades, but be patient and enjoy the scenery.
Beals Dock in Southwest Harbor has the best lobster rolls I've ever eaten.
Don't order BBQ anywhere in Maine. Never. Ever.
If you want to know any specifics, let me know.