Answer: You didn't mention how deep the planter is. That information is important as some perennials have very deep root systems and others have shallow roots. If it's at least 12" deep, you can plant most any of the following: Lavender, basil, thyme, sage, dill, chives, lemon verbena, mint, oregano, rosemary, and parsley can be combined with flowers such as nasturtiums, violas, zinnias, portulaca, scaevola, geraniums, and dahlias. These herbs and flowers need full sun, which means at least 6 hours a day. Choose an all purpose potting soil that contains some humus (not just peat moss) and moisten the soil.
For even more ideas, let me recommend a great book on the subject: Container Gardening For Dummies by Bill Marken and the Editors of National Gardening Magazine (for more info, check out http://www.dummies.com). This takes you through the whole process of design, planting, care, etc. Best of luck and have fun!
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