I would probably go to papercrafts.com. They have ideas posted. Also, you could try taking a piece of cardstock, cutting it to 5.5 x 8.5, fold it in half. Decorate it like a scrapbook page. Use stickers, embellishments, ribbons. Use patterned paper and have it fill half of the card. Use another cardstock as a strip and write Happy Birthday.
Hope that helps. They don't have to be too complicated.