If you're looking to saying it with flowers, you may want to consider their language. Different flowers have traditional meanings that have mostly been forgotten.

Roses may be the exception. Red roses express love and romance. Pink roses can mean love gratitude, or appreciation. White roses, often in a bride's bouquet, signify marriage, spirituality, and new starts. Orange roses express enthusiasm and passion, while yellow roses mean friendship, joy, and good health. So, if you expected to get red roses from that certain someone and were presented with yellow, you may want to "have a talk"!

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Other flowers have their meaning, too. Asters are a symbol of love and daintiness; yellow carnations mean rejection; geraniums signify stupidity, and calla lilies mean beauty. And if you receive a bouquet of withered flowers...well...rejection. (Sorry)

With a little research you can have fun writing your secret message with a bouquet of flowers.