You could use a photo and add some scrapbook paper and ribbon and mod podge or use a glue stick to glue the magnet onto the back. Or I have also made a little card with my information and put the magnet on the back and given it to the women that I visit teach.
Another way I have used them is as a bookmark. Take a strip of stronger scrapbook paper or card stock, fold it in half and glue the magnet on the inside of each end of the scrapbook paper, so they will pull together. You can then fold it over the page you are on in a book and it will save your page for you.
There you have it some easy super thrifty ways to reuse those advertisement magnets.