Do you have any sort of photo editor on your Mac, like Photoshop? If not, you can download Gimp for free which does the same sorts of things. It is going to be a bit more intensive than Paint for instance, but will get you a nice looking certificate if you play around with it long enough.
Here's the link for Gimp
http://www.gimp.org/downloads/
So what I usually do is find a basic blank gift certificate image in google and then just edit it in your graphics editor. You can use this for a lot of things from gift certificates, achievement certificates, awards, gifts, etc.
HTH