I've used on walls and it doesn't remove the paint or hurt the finish. I got them at a teacher learning type of store. I'm sure you can get them at wal-mart or some sort of art supply store. I know you or some other creative people could help with exactly what metal object, such as the side of a big filing cabinet. I personally do not like to clutter my refrigerator and could understand why no one else would want to.
Kids love all kind of magnets.
Collecting them as well as what they hang with them. You can get big strong ones for those heavy projects. There are so many different kinds and you can always make projects with magnets as well. My children always enjoyed playing with the magnets. They would arrange little scenes, or if the magnets were cars, a pretend road was made.