I do NOT have a green thumb at all and I seem to kill most flowers. BUT, impatiens/geraniums/hostas are the easiest to grow! Also, check out your local garden nursery and get suggestions there from the informative folks. They will know your zone/weather conditions and can give you the best recommendations.

My hostas and ferns thrive in my shaded areas of the garden. Here's a list of other suggestions:
Shade loving plants

Also, you may want to substitute the planter boxes with container gardening. I noticed that planter boxes dry out often, where a deep pot/container seems to do better for the flowers.

Good luck! I'm hoping not too kill too many things this year in my garden.