Castine (on the Penobscot Bay) is my home port. The entire bay is filled with excellent cruising destinations. The advice in this thread has been excellent. There are many island to explore and wonderful, safe anchorages to spend the night.
I'd add two warnings to what has been written so far:
1. Make sure you understand the architecture of how lobster pots are put together. I do some fill-in diving for TowBoatUS here cutting pots off the rudders and props of boats cruising through. Lobster pots are easy to avoid but you have to know what you're looking at or else you'll get snagged.
2. The Maine marine patrol is doing a little too good of a job this year. They now have spiffy Protector boats zipping around. We recently returned home from cruising from Florida to Castine and weren't bothered until we were boarded by the Maine marine patrol on the Penobscot Bay. A Maine boarding is much simpler than a Coast Guard one. Just make sure you have proper registration/documentation, PFD's, and flares. It's all they look for.
It's looking to be a nice Summer here on the Penobscot Bay this year. Come visit us!