aromaticwings - you want to look for drought-tolerant plants like portulaca, petunias, vinca, marigolds, etc. People can come up with a million different combinations for all types of growing conditions. So it's one of those things where walking into a nursery and checking out what they have and mixing and matching based on the amount of sun the plants can take, as well as looking at foliage type and plant heights, can sometimes yield a unique combination that really works out well. It's experimenting! There are many sites out there that show combos but one of the Farmer's almanac sites has a nice simple list of popular plants grouped by exposure and type of habit that is helpful! In the past, I usually planted for spring (eg., pansies, snapdragons,etc) and then summer (where the summer plants could go through to frost - eg., petunias, wax begonias, sweet potato vine, etc). But this year I'm trying some Salvia coccinea "Hummingbird Red" and "Lady in Red" to give my hummers something to buzz around. I surrounded them with some white alyssum for fragrance (although I know they tend to go a bit dormant when it gets hot but are wonderful in spring/fall or cool days in summer).

Is this a real window box, hanging below a window or is it on the ground? If it's hanging I think you'd have to stay with minis, and small ones at that. If it's on the ground a full size rose should be OK. But even then I think you'd have to keep it on the small side because of the size of the box. The problem is knowing how big something is going to get in your climate. You should probably check with a local rose society to see what they would suggest for your area.

People will need to know where you are located, especially if you want something perennial.

Geraniums are great. But flower boxes are very needy. I love gardening but flower boxes are too needy for me, especially in full sun. They usually need water every single day. FYI: pansies are a cool season flower. They are good early in the spring and mid-fall. In between those times: no. 70 degrees is about their limit for heat.