Writing a song is such a personal adventure. In a lot of ways simply the act of writing a song for a loved one or friend will probably get you in their good books for most of the new year.
If it’s a Christmas-themed song you are after you may wish to employ a bunch of major-, minor-seven chords---the very dreamiest of chords. Also straight seven chords work nicely to add a folky, warm campfire sound.
Lyrically you really need to work in the person’s name (or pet name)---it sounds cheesy, but it works! This exercise has already been done by the best songwriters in the world. Take cues from the pros: throw in French words to sound romantic and educated (“Michelle” by The Beatles). Use general language to potentially reuse the song (“Let’s Get it On” by Marvin Gaye). Get a little rough around the edges with the lyrics to show you are a wild and passionate (“Alison” by Elvis Costello). Or get prog’y on that song and impress the giftee with mixolydian scales on the guitar (“Rosanna” by Toto).