I've been loving yellow lately. Here is a cute birthday party from one of my favorite blogs, Hostess with the Mostest. The cool thing is that you don't have to use it just for a baby's birthday party, but you can adapt the idea for other events as well.
Buy a cheap frame from a thrift store and spray paint it. Easy! They sell letters at craft stores for a couple bucks.
Use scrap fabric from old t-shirts or sewing projects. Again, so simple!