I'm not in your zone, but this is a lovely idea. Some plants that come to mind for a fountain look are lavender and ornamental grasses in the lower tier. Lavender with a long wand, such as Provence or Hidcote might be nice. Lavender loves to be dry and well-drained, full sun. It attracts honey bees and butterflies. The flowers can be used for home products, crafts, etc. It smells wonderful. Lavender is probably perennial in your area. Most varieties are for me and they are evergreen for me.

In the 2nd tier, you'll probably want a cascading plant. I can't make a recommendation for your zone, but this may be a good place for annuals for constant color or reliable perennials with long-bloom seasons. Homestead Verbena is purple, perennial heliotrope is a lavender/blue; lantana can be had in a variety of colors; wave petunias or million bells; just some thoughts.

You may also want to ask on the Container Gardening Forum (I think there is one?)

Cameron (in North Carolina)