used to be in the Middle Ages, you can reach his/her sweet ears using loudspeakers and amplifiers when performing your small rock or hip-hop serenade.
Another idea is transmitting your Saint Valentine's serenade of love online using a webcam or via radio or TV or by recording it on a CD or VHS.
You know, basically you should not necessarily compose a masterpiece of music worth Mozart's clavier, or a sonet worth Petrarca's quill, but simply a serenade serious enough not to be considered mockery and demonstrating that you took into account his/her musical preferences. If your beloved is fond of HIM, put on a make-up like his and sing one of his songs, if she/he prefers Black Eyed Peas or Bob Marley, try to imitate their manner of singing, if she/he likes listening to tribal music, disguise yourself as a savage and try some drumming etc.
I assure you 100%, he/she is going to enjoy this serenade very much no matter if you play well or bad, because HE/SHE LOVES YOU!