Four o' clocks have great evening fragrance (after 4:00) and are easy, at least for me. There's also evening scented stock and moonflower, although the latter is a vine. All are annuals in my neck of the woods, and all are night fragrant.

You didn't say how much sun your window box gets. Or how much height you want plants to have in there.

Some petunias are very fragrant and would look pretty cascading from a window box, as long as it gets enough sun for them to be happy.

Are you tied to the idea of the fragrance coming from flowers? A window box is so small to have enough of a mass of flowers for wafting scent. Something like basil or a mint might be more rewarding. When you open the window, ruffle the plants, snip them a little. As long as the wind is blowing in that window, I'm sure it would scent the room, but it would be savory, not flowery, if that would appeal to you. Just walking by basil growing in the yard, you can smell it. A little globe basil might work better in a window box but it wouldn't be hard to just keep cutting a regular one to keep it from getting so tall it blocks the view, especially if you like to put it in food as much as we do.

With an herbal scent, you don't have to worry about whether or not the plant is making flowers to get the scent, and I think a lot of the herbs have much stronger scents that waft well.

A general idea might be to use cache pots, so you can shift different plants in/out, depending on which one has something to offer at the time. Like, have the next seasons' plant already bought, getting bigger... Easy to change on a whim without disturbing roots, and allows you to bring something inside easily if you want to try saving it over winter. Might dry too quickly for some people's schedules though...