Even though you can plant all summer, May is the last "best" month to get annuals, perennials, trees and shrubs in the ground. Annuals take about a month to get established while trees and shrubs can take several months so don't delay!
Many plants are just starting to bloom in May. Hydrangeas, tropical hibiscus and dahlias will be in full color by now.
May is also a transition month for watering. Plant water use will start to jump up as the days become longer and warmer. A good topdressing of mulch or bark in very important to keep soil moist and reduce plant stress.
Instead of planting flowers everywhere try hanging baskets or large pots strategically placed throughout the garden. Colorful or dramatic pots add interest and uniqueness to any garden. Don't get one too small, 18 in. wide minimum. You can hook them up with drip adapters off sprinkler heads for convenient watering.