Personally, I don't think there's any reason for doing this. Almost nothing to see between Bangor and the Canadian border (120 miles), and very few nearby geocaches. The very nice
Kings Landing Historical Settlement near Fredericton, New Brunswick is definitely worth a visit, though (1 hour beyond the border).
I'd recommend traveling along the coast to the Canadian border. Great scenery, lots of geocaches, and nice towns at the end of the line: Calais in the US, St. Stephen (ChocolateTown Canada) across the border. And you can take the fairly fast Route 9 (2 hours or so) back to Bangor.
And I don't remember if anyone mentioned, but don't forget that you need either a passport or passport card to get back into the US.