Are magnets muktzah, or may one play with them on shabbos?
It is permitted to play with magnets on Shabbos, and they are not considered muktzah.
Sources: There is no melachah involved in magnets, and provided that they are meant for a permitted use, such as a game, there is no prohibition of using them on Shabbos. Some have raised the issue of sticking a magnet on a fridge on Shabbos, because the size of the fridge would consider this an act of adding to a fixed construction. However, because the nature of fridge magnets is to take them on and off often, it would appear that this would not constitute a problem. See Rema, Orach Chaim 314, and Or Le-Zion, vol. 2, no. 208, who discussed attaching something onto a cupboard or fridge by means of a vacuum.