One thing I would recommend is different kinds of sweet potato vine. They have a lovely chartreuse color, and a dark purple. They would trail over the window box. Not sure if this can take full sun, also not sure if can WS, but why not?
Not sure the exact name, but calibrocha? Million bells? I love verbena. I am thinking of flowers that would trail over. Even small mini sunflowers I would just love!

Hi pameliap, I'm doing the same thing but with a few smaller boxes. I've seen some really nice pix for inspiration, and like louisianagal decided to use sweet potato vines. I will have both colors, and also dichondra silver falls, and then red geraniums in the center. Later in the season the geraniums will get replaced with Vinca Jaio Red if they can't stand the heat... Also sowing some nierembergia, which I've never had but is said to look delicate but be very tough through the heat. They are white with yellow centers. Another trailing plant I love is blue daze (evolvulus), bloomed nonstop right through november for me, some days it had 60-70 blooms on it on a hanging basket, and withstood erratic watering very well. I will definetely have it again. Finally, I am not sure if they are trailing (growing for the 1st time this year), but if you like Petunias, there is the Laura Bush variety that is supposed to love heat and bloom right through it. It's only available through wildseedfarms.com, I just got a very nice order from them. I believe they are purple but now have pink as well. I am including a couple links I found especially inspirational when I was in the design stage of my planters. Good luck and have fun!
Leticia
This one is for Hooks & Latice's contest, lots of pics
http://www.hooksandlattice.com/windowbox-contest-2007.html

The one below is for someone's balcony garden blog, which I also really like

Here is a link that might be useful: Citygarden's blog

That's a nice big windowbox! You could get a lot in there -

I've picked up a few ideas for windowboxes over the years - treat the windowbox like a mini garden bed and the possibilities are endless!

Some plants that would be useful for your sunny spot (some might be perennial in your zone; you could move them to the garden at the end of the season or just treat them as annuals):

Trailing lobelia (lobelia erinus)
Sweet alyssum
Pelargonium
Marigolds (petite)
Flowering maple (a "dwarf" variety would be best)
Marguerites
Potentilla
Bidens
English ivy
Salvia
Vinca
Cascade petunias
verbena
Bacopa
Nastutiums
Calendula
Lantana
Lavender
Schizanthus
Draceana (spikes)
Coleus (sun-tolerant)
Blue-eyed grass
Sedum (some of the mini ones)
Dianthus
(herbs can be nice too; if they have a fragrance, that's even better, imho)
Lemon thyme
Basil
Chocolate mint
Fernleaf dill

And, of course, the sweet potato vines, as mentioned above, with one caution - watch that chartreuse one! It trys to take over the world!

There's lots more you could use - just open up your imagination!

HTH -

PV

Great ideas - thanks and keep 'em coming!

I've been seed shopping already (any excuse will do, lol) and have ordered Angelonia (3 different colors), Alternanthera, Laurentia, Chaenarrhinum, Nierembergia, and Lobelia, plus an ornamental grass Melinis Savannah. I'll probably pick up some Sweet Potato vine and Calibrochea locally in the spring.

I already have the seed for many of the other suggestions and they're on my winter sowing list, so I can have them ready to pop into the box if the original plants don't work out, or to change out the look mid season.

Since it is a large "box", I think I'll line it with either moss or coir, and keep the plants in separate containers to make it easier to swap them out if necessary. The area gets very hot, so however I plant them, I'll be watering daily.

I think you could direct sow it with petunias, alyssum, portulaca and then transplant in some spikes later on after it germinates.

My sister has a windowbox that roasts so fast the soil dries out in almost a day, she solved the problem by running a soaker hose over it and attaching the hose to a timer.