Magic Ink
Every romantic has a little magician inside. Although the former talks about surprises and the latter about tricks, both are eager to captivate their audience with the best of their art, and both enjoy it immensely when everything goes as planned. With so much in common, surely it's possible to add a magic trick to our romantic repertoire that allows us to remind our partner how special they are while leaving them in awe.
The trick we'll borrow from the magicians' guild is magic ink. There are two very spectacular effects you can achieve:
Invisible ink that becomes visible: it's a substance that isn't seen on paper but appears when a certain action is performed. The classic example is lemon juice. Write something with lemon juice on a paper, let it dry, and then pass a match underneath. You'll see that what you wrote slowly becomes visible.
Visible ink that becomes invisible: the effect is the opposite of the previous one. This type of ink can be purchased at magic shops for about 10 euros. In this case, you usually draw with conventional ink what you want your partner to eventually see, and then camouflage it as best as possible with the magic ink. The result should be an unintelligible jumble that can only be deciphered when a heat source is brought close, making the magic ink disappear.
With these two effects, you can achieve great surprises. For example, imagine a letter with a series of letters and drawings that seem to be from another planet and with the following legend in Spanish: 'Only those who illuminate the message with the fire of their love will be able to see beyond.' If our surprised one is even slightly perceptive, they'll bring a lighter or a match to the text, which will suddenly reveal the message you left hidden under the magic ink.