Royal icing is so much fun to work with! I like how smooth and shiny it looks. Here are some autumn-colored cookies I decorated recently with royal icing.
For the hearts, I piped short orange and horizontal lines onto the red icing while the red icing was still wet. Then I dragged a toothpick through those lines, creating the hearts.
The polkadots are really fun to do! You have to work quickly so the first layer of icing doesn't dry before you finish piping your dots. Use a thinned royal icing to add the orange dots while the red icing is still wet. The dots will settle and spread, creating a smooth, flat surface. For the sort of houndstooth looking one, you pipe orange lines onto the wet red icing, then drag a toothpick through the lines (first from left to right, then right to left).
The great thing about royal icing is that it looks so smooth, and after you let it dry for a few hours, you can stack your cookies without ruining the icing.