What I do with my window boxes is plant/start annuals (right now dwarf sunflowers and nasturtiums but you can check a local nursery to see what does well in your climate) in a series of square nursery pots (I can fit 5 in each window box). That way I can switch out one or all of the plantings if something fails to thrive, gets played out, or the critters get to it.

I like annuals for the quick shot of color and the fact that window boxes are not really deep/big enough for perennials.

I'd recommend portulacas/moss rose or gazanias. They are both sun and heat lovers, and they don't have deep roots.