Just like you can make and frame your own chalkboard fairly easily these days, you can also make a magnet board. Sure you could go out and buy one of those magnet boards meant for college dorm room doors, but if you make it, you can customize the look to fit right into your kitchen.
Instead of using your fridge, you can put it in a safe place away from the oven so the kids can play while you cook like Kelli did.
Kelli purchased a sheet of metal and cut it to the size she wanted, adhered it to a cabinet and stuck a frame around it. It fits right in and her three boys are enjoying it.
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I've done the same thing, sort of. I painted my sheet metal with plans of hanging it on the wall in our long hallway. I'm still waiting for my dad to come by with his drill to hang it up for me.
I made something similar. Picked up a large picture frame from the thrift shop, cut sheet metal to size, painted the sheet metal with chalkboard paint and hung it on the wall. Now we have a magnetic chalkboard. Works great.
I made a magnetic chalkboard using a sheet of steel too: http://1smootshort.blogspot.com/2010/01/weekend-project-number-three-magnetic.html
I did not make one but did attach a framed magnetic white board to the side of our kitchen island using the 3m velcro strips. This way it can be removed, is still in the kitchen, and does not scratch our fridge.
a baking sheet is a portable option. i used them in my 1st grade classroom and the kids could take them anywhere.