First, on the gardening maintenance scale, nearly all perennials rate pretty high up, right after annuals. For low maintenance, look at woody plants - shrubs and trees. And if possible, evergreen ones or those that hold their foliage all year. These can offer flowers as well and many can have colored foliage to increase the visual draw.

The other big plus with trees and shrubs is that they have a permanent presence. In cold winter climates - like yours - there will always be something there. Perennials die back to the ground each winter and so you have an empty bed for 6 months out of the year.

For sun you can look at dwarf lilacs, spirea, Rose of Sharon, butterfly bush dwarf weigela. For shade consider hydrangeas, dappled willow, clethra.

For shade, ferns and hostas are great accompaniments. Hostas are perennials but about the lowest maintenance perennial I can think of - these are truly plant and forget plants. Otherwise, perennials typically need a fair amount of grooming and deadheading, an annual cutting back or clean up, periodic division, staking, cautions about self-seeding and aggressive spreading.

Hardy bulbs also tend to be very low maintenance, especially the fall planted, spring blooming types.

I'm not in your zone so would not hazard a guess as to what might work for you, but just wanted to let you know that there is a Maine Gardening forum which you might find helpful as well as the New to Gardening Forum. Good luck with your project. You'll enjoy it.

Here is a link that might be useful: Maine Gardening