I love lavender but think of them as large bushes. Will they do OK in small window boxes?

I have seen some dwarf lavenders offered that are supposed to only reach 10-12" in height, but I have not grown them myself.

I would go for Carnations temporarily.
Check out Daphne 'Eternal Fragrance' or another D. x transatlantica hybrid. I am not too crazy about the scent but you may like it (since scent is a personal thing). The reason I recommend it is because it is suited for containers and doesn't get terribly huge 2-3 ft. And most importantly, it blooms intermittently throughout all seasons.

First thought...the size of those window boxes, especially in sun...need to shade the roots somehow, keep the soil cooler and I would have a couple of trailing plants (even some kind of ivy because they're tough and could take the situation) in them also to hang down and shade the boxes somewhat.....seasonally, species freesias (the white ones that are most heavily scented). You might have the couple of trailing plants and 2-3 plants in pots that can be put in and taken out for seasonal display and because that way they would not have to deal with the situation permanently in which they might decline.

How about jasmine?