Charming and very practical
A popular window box style for colonial houses is the hayrack style. You've probably seen pictures of these attached to the windows of European houses. Since they are open structured, things are likely to plop right out of them unless you line them with sphagnum moss and use pots to hold the plants. Other than hayracks, window boxes tend to be enclosed containers.
If your window box is too beautiful to endanger, then use it as a holder for multiple small pots of plants. But if it's sturdy and practical, make sure you have drainage holes in the bottom and some wire mesh to keep in the potting mix, and treat it as a miniature garden.
The key to a really good-looking window box is proportion. A good rule of thumb is to make them 25% of the height of a short window and 20% the height of a tall one. Too skimpy and they'll look like badly applied false eyelashes.