Hi Pattycake --
I think a regular royal icing (confectioner's sugar & egg white) thinned to make it "floodable" would be what you'd want. It couldn't be easier, and it's perfectly safe at room temperature because of the proportion of sugar to egg. This is the icing that pro's like Toba Garrett use for sugar cookies.
If you're going to pipe fine designs with the icing, you'll need to sift your confectioner's sugar first (a good idea in any event, even though it's a pain -- it tends to get all over everything I've found!).
Either way, let's say that you want to to a heart-shaped cookie really neatly. You make up some royal icing to a pipeable consistency and pipe an outline. Presuming you're using the same color for the inside, you then thin the same icing to a "floodwork" consistency -- that means with a bit of water or lemon juice, you thin it until it is such that the line you make dragging a knife through it closes over again by itself within a couple of seconds. Does that make sense?
When the icing is that consistency, you can easily pipe it into the center of your design fairly loosely and quickly, and it will even itself out beautifully, drying to a smooth, glossy finish.
If you want any other info, please let me know!
Edited to add: once this icing is dry, the cookies are stackable which is hugely helpful!