If you're near the coast, head to any of the coastal towns for major inspiration. They do some seriously beautiful containers. The one plant I personally AVOID for window boxes is bacopa, and it is sold everywhere. The reason I don't like it is because if the soil is not kept moist it will put out lots of greenery and no flowers. It looks perfectly healthy as far as green goes, but no flowers. If a summer is especially hot, it is a real challenge to keep them moist enough, and the fact that they prefer cooler weather only adds to the frustration. Because they don't wilt or give signs that they're too dry, you don't realize it until the flowers disappear. That's my personal limited experience, but maybe somebody else will chime in. If you have a container that has other moisture loving plants and is in a place that won't get excessively hot (as in not on a brick patio or by a wall heated by the sun) then it would probably do better. Sweet alyssum can get "overdone" sometimes in containers too. All the Supertunias do extremely well. If you want it easy, find some Supertunias you like and then add some greenery for interest, like Silver Falls Dichondra. Then head to the tourist areas for some inspiration for next year.

I don't know if you've done flower boxes before, but just in case I'll tell you one of my biggest mistakes starting out. I put too many flowers in the box. As the flowers and root systems grew, there was not enough soil in the box to keep the flowers happy and moist. They did not do well. Now I try to be sure to not stuff the box full (which is so easy to do when the flowers are small) so that as they grow there will still be soil in the box to hold water.

In case you read my above post earlier, I've edited it. I originally incorrectly said lobelia when I should have put bacopa. Lobelias are fantastic in containers, a favorite. It's bacopas that seem to be sold everywhere but have been a pain because they have to have consistent moisture to flower. Sorry for the confusion!