This is what you should write in a birthday card when you have no idea what to say:
"Do me a favor. I couldn't think of anything to write, so please pretend that you're reading something really touching, maybe even wipe away a tear, and then look at me and say, "That is so beautiful. I didn't know you could write like that." Then if anyone asks to see the card, refuse and tell them it was just too personal. Thanks, I owe you one."