Half the time when I'm making buttercream type frosting, I go with melted butter, and just mix icing sugar in to taste - forming a kind of paste or glaze consistency. It has a flatter, less fluffy consistency this way, instead of doing it the "right" way with whipped butter - but I am usually more concerned about the flavor profile, especially if its for casual eating rather than for guests. Its quick and easy and foolproof, this way, and easy to add make just a little or add more if running low, since you don't have to fluff ingredients separately.
Other than that, you can make a glaze quickly by making a thick liquid or paste of the powdered sugar with any reasonable liquid (or, yanno, unreasonable ones if that floats your boat). Water is easy, as ann mentioned, but you can use milk, or juice if you want a quickly flavored glaze, melted chocolate, any syrup that strikes your fancy, whatever.