To choose a light-hearted love poem to send to an ex-boyfriend, look into poems by Ogden Nash or e. e. cummings, depending on how overtly you wish to state your love. Both poets are famous for their craft and whimsy, but differ greatly in their impact.
Ogden Nash is famous for his short, sometimes pithy poems, which are precise in language and often laugh-out-loud funny. If the desire is to impart a playful attitude, this would be a good poet from which to choose. In particular, read his poem "To My Valentine," which can be quoted by the verse or in its entirety. If a simple, silly poem is all that is required, read Nash's "Reflection on Caution" or "The Octopus."
e. e. cummings (spelled just like that, without caps) is famous for throwing away the rules of grammar and punctuation and reinventing them to lend import to his works. They end up looking playful and light, but often convey heavy emotional weight. His poem, "somewhere I have never traveled, gladly beyond" is often read at weddings, and therefore should be avoided if attempting light-hearted tone. Read instead "maggie and millie and molly and may" or "2 little whos."
For a personal touch, send the poem handwritten into a card or on stationery, and include a brief "thinking of you" statement before your signature.