Have you tried coleus? They don't flower but the foliage is very colorful.

I have found window boxes impossible to deal with. They are under the eaves of the house so they must be watered even when you have had lots of rain. They are frequently in the deep shade. Most boxes are too small to be effective. My squirrels love to plant walnuts in them, and the neighborhood cats love to sit in them. I always wanted to have a house with "those cute little widowboxes", but never again will I have that desire. I have already removed some of mine. My best year of growing in them was a begonia plant, but I replanted them many times that year because the squirrels dug them up frequently.

thanks for tip on the coleus, I had not thought of planting them and there seems to be a lot of colorful choices. I just bought some orange ones called sedona and some red ones called big red. They both said "sun or shade". Has anyone ever had luck in shaded window boxes with them? I hope that my $40 investment pays off this summer!

Geraniums.

I agree with the idea of coleus. I plant them in pots and window boxes all over my porch and they thrive beautifully in the shade and hour or so of indirect sunlight they get every morning.