You could probably put just about anything in them, particularly those that like part sun or shade. I'm not a connoisseur of design but for my morning sun windowboxes, I have planted various things over the years like begonias, geraniums (pelargoniums), alyssum, petunias, annual salvia, sweet potato vine, vinca vine, impatiens, verbena, snap dragons, and pansies (the latter 2 more for spring). I've probably had other things in them but don't remember right off. Currently I've been planting hummer plants in them. I currently have separate little pots of plants in hangers along my rail including fuchsias, pentas, basil, oregano, firecracker vine, agastache rupestris, portulaca, and some other odds and ends.

If you like sun lovers like marigolds or portulaca or say the sweet potato vine 'Blackie' (which is the dark-leaf form), etc., they may not bloom as much and the foliage of the vine would probably go greenish, but then depending on how much light manages to hit the windowboxes, they may do okay and will appreciate cooler conditions.