Looking for additions to my sourdough bread

So I am now getting to the point where I can consistently make a good loaf of sourdough with just flour water salt and starter. I absolutely LOVE this simple bread, but I'm looking to branch out and incorporate other ingredients into my loaves (stuff like fats, nuts, oats, fruit, etc.) I see so many delicious recipes online, I'm just not sure where to start. Anyone had any suggestions for good "beginner" additions?