Once these have flowered don't throw them out. Cut off the heads (unless you want seed) then put them somewhere that the leaves can get the sun. This will feed the bulb for the next year. Once the leaves have died you can plant the bulbs outside and they will flower at the normal (unforced) time next year.
The narcissus Tete-a-tete is particularly good, and provides early colour and a delicate fragrance too.
Below I have listed groups of plants. I have tried to include at least four plants in each list as you may not be able to find all of them, although, unless you have a very large windowbox, I would recommend that you have just three in each box.
praecox, wild thyme
Thymes make a very fragrant, easy to care for windowbox, and an excellent choice for windy sites. The flower colour will be pinky/purple, and you can eat the leaves if your air is not too polluted. Try to get one variegated thyme to add a little colour when there are no flowers.
Sage, mint, chives, thyme, rosemary, lemon balm, tarragon, summer savory
Get the plants from the herb section of the supermarket, so you can eat the leaves. Do not include basil as it needs greater fertility than the others, however it can be included if you put it in a pot in the windowbox and give it more fertile compost or feed it more often. Pot the rosemary up separately if it grows too large. French tarragon is tastier, but more delicate than the Russian tarragon if you intend eating it. If it is purely for the insects, then the Russian is hardier.
longifolia, horse mint
Mints are fairly fast growers, so you could start this box with seed. They are thugs, though, and will very soon be fighting for space. So you will either have to thin and cut back or else you will end up with one species - the strongest. The very best mint tea I ever had was in Marrakech. A glass full of freshly cut mint was placed in front of me, and boiling water was poured into it. Then I was given a cube of sugar to hold between my teeth while I sipped the tea. Plant this box and you can have mint tea for months.
Too many to list
For year-round colour try to plant varieties that flower at different times of year. Heather requires acid soils, so fertilise with an ericaceous fertiliser, and plant in ericaceous compost. Cut back after flowering and remove the cuttings. It is best to buy plants as heather is slow growing.
This will give you flowers from March till July. The bluebells should be bought as bulbs, as seed will take a few years to flower. The others can be started from seed. Forget-me-not tends to get leggy and covered in downy mildew after flowering, so is best hauled out to give room for others. Alkanet can be a bit of a thug, but will grow in shadier, damper places. In fact it can brighten up a dull corner, so is worth it.
vulneraria, kidney vetch
These will give you flowers from May to October, and if you include the primrose, from February. Try to include a vetch as they can climb or trail so occupy the space that other plants can't. All can be grown from seed.
repens, white clover
All can be grown from seed. The clover and daisy will have to be cut back as they will take over. The clover roots add nitrogen to the soil. The mignonette flower doesn't look very special, but the fragrance is wonderful, and the alyssum smells of honey.
flos-cucli, ragged robin
The comfrey will try to take over. Its leaves make an excellent fertiliser, and are very good on the compost heap, though windowbox gardeners rarely have one.
Lonicera spp., honeysuckle
The sweet pea will need twine or something to climb up, so is suitable if you have sliding windows or window that open inwards. You will be rewarded by a fragrant curtain every time you open your window.
Spring bulbs and late wildflowers
Aster spp., Michaelmas daisy
The idea of this box is to maximize your space. The bulbs (cyclamen has a corm) will flower and do their stuff early in the year. After flowering cut the heads off as you don't want them making seed, but leave the leaves as they fatten up the bulbs to store energy for next year. The foliage of the wildflowers will hide the bulb leaves to some extent. Then the wildflowers take over and flower till autumn.
Related pages, EUROPEAN FLOWER LIST-NORTH AMERICAN FLOWERLIST - window box main page, window box what to grow and where to grow it