For my friend's birthday I got them a Lovely Cake
(it actually shows up in-world), and shot off a bunch of fireworks. If that someone is special and you still have it sitting around your inventory, have them join you for a picnic with a Romantic Picnic Basket
If you're looking to send gifts in the mail, you can try your luck at Blingtron and mail them an Eternium Rose
and any mini-pets that they don't already have.