I read about making your own notepads with padding compound on the most excellent blog, the small object steno pad, and immediately felt I had to run out and do it myself. Padding compound is a fancy word for the red stuff at the edge of notepads that keeps the pages stuck together. Turns out you can buy a huge jar of it for cheap (I ordered mine from amazon), and all you basically have to do is pinch some pages together and paint the compound on the end, then let it dry. For a full tutorial, check out the above link to the small object steno pad.
I acquired (by request) some defunct letterhead from my granddad, who was downsizing. Then I carved a fish stamp using an eraser and lino-cutting tools, then stamped each page with it. Next, I saved cereal boxes for the backing.
I read in the comments on the small object steno pad that an easy way to do a bunch of pads at once is just to layer all the paper and cardboard together and paint the padding compound on the whole batch, then separate the individual pads with a knife.
To keep the pages together tightly, I pressed with clamps between two pieces of scrap wood. Voila!
One tip on using the padding compound—mix it up thoroughly with a stick or spoon before using, as otherwise it will be too watery to work.
Lastly, I dug up a stack of commercial magnets that I had been saving for just such a need. I glued a magnet on the back of each pad so they could hang on the fridge.