One of my favorite wet-on-wet looks is marbling. Back in my last life as a teacher, when I was single no kids and had a TON of free time I once took a paper marbling class. The methods are the same for cookies, just a different medium. Here's how I decorated a couple of the M's for my nephew's birthday earlier this month. I'm using all flood icing here of the same consistency, about 18- to 20-second icing.
One of the keys to this method is to work fast. I wouldn't advise doing more than one cookie at a time, especially if you're doing larger cookies.
The above would be a perfectly lovely marbled cookie and you can stop there for one look. You can vary it's appearance by making the lines with the toothpick closer together if you like. Or...
This method can be used in so many different ways and on so many different shaped cookies. You can achieve a different look by changing the order in which you make your strips and varying the tip size that you use to make your strips for different thicknesses of lines.