Find fake flowers at a thrift shop, rinse them up in the bathtub if you don't have a kitchen sink sprayer, and cut them up to hot-glue around the brim.
Ribbon and silk flowers and lace have always worked well for me. Just gather up what you can lay your hands on, and try out various ways. Once I put a large rinestone pin on my garden party hat, and that was a big hit, and it was real, real simple.
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