Hello, I grew up in Maine and worked in Bar Harbor for a summer. The park is beautiful, and the town isn't all T-shirts if you know where to look. Don't be afraid to explore.
If you have time for a movie or want to take your friend out, hit up Reel Pizza, they are a local movie place that shows a good mix, with couches, fresh pizza, and an intermission. For lunch you should check out the burrito place, Gringo's, also fresh made and very filling. Siam Orchid is a nice Thai place run by a kind Thai family, but they can be a bit expensive.
It is still 'off season' until May there, so a lot of places will be closed. Though right now we are just getting out of a warm snap, so there is almost no snow anywhere. On the coast be ready for windchill, fog, and moist conditions.
There are mosquitos and ticks are out because last week was unusually warm, but they aren't very bad yet and the cold weather should keep them down.
Temps right now range from the 20's to the 60's depending on the day.
Dogs are welcome there, many places let them right in without problem, though it is always nice to ask first. Your medical dog shouldn't be a problem, the local area is a bit dog crazy.
For camping, there are a lot of camp sites, as camping in the park is discouraged and much of the island is private property. If you endup looking for a hotel, 'The Grand' is right in town and usually has cheaper off season rates. For camping, bring your warm insulation, again because the temp drops into roughly the 30's at night, and the town is an Island, so you get coastal winds in a lot of places and always be ready for wet weather.