Here is a quick and simple way to make a magnetic board out of just a cookie sheet (thanks to Bold Bright and Beautiful!). Baking sheets are cheap, and come in different shapes in sizes to fit your taste. As long as it's not stainless steel or aluminum, it will be magnetic!
It's really up to you how you want to decorate it - try adhering patterned contact paper, spray-painting it, modge-podging photos onto it, covering it with cute fabric, or leaving it bare and adding a cute frame around the edge. You can drill holes in your sheet to put ribbon through for hanging as well.
Also, you can easily make your own magnets by taking earrings, buttons, or stones and glueing plain magnets on the back. If you want a vintage look for your board, scour antique stores for old-school baking sheets that come in cool patterns or have scalloped edges. No decoration needed!