Poetry of Your Own
Try to compose a verse or two of your own poetry to include in the card. You do not have to be a published poet. There are many writing exercises (see reference section) that will help you write something lovely and original. Just remember two things: Poetry does not have to rhyme, nor does it need to be about love and romance to be romantic.
If you feel completely lost when faced with the idea of writing poetry, it is perfectly acceptable to borrow the lines of a luminary. Find a poem that speaks to you and bears some significance for you and the card's recipient. Love poetry should not be saccharine.
A good place to start looking might be with e.e. cummings' "i carry your heart with me," or Li-Young Li's gorgeous poem "From Blossoms." Visit respectable poetry websites (see reference section) to find lists of poems organized by theme. You don't need to quote the entire poem; just a stanza or two will do. The poem does not even have to be about love; if there are a few lines that are meaningful to you, excerpt those. Be sure to give credit to the poet; the romance vanishes when the idea of plagiarism comes up.
You can hardly fail when you write directly to the person of your affections. Listing the reasons why he or she is beautiful is a bit overdone, but consider other types of lists: moments that were beautiful or moments that were funny or tender. In lieu of a list, focus on a moment or day and write it in the second person (you) for an intimate effect. The time on which you focus does not need to be dramatic. In fact, letting someone know you noticed or felt something during a quiet moment can communicate a strong emotion. Both the lists and the description let the other person know you are paying attention to them.