Some additives that you may already have in your house.
Honey - start with 1 teaspoon PPO (per pound of oils). This may cause your soap to heat up, so keep an eye on it!
Spices - cinnamon, clove, anise, nutmeg. They add an interesting speckled look and a nice (very light) scent.
Dried parsley - interesting green color (will slowly fade)
Fruits and veggies - pumpkin, avocado, banana, carrot, tomato paste. Do some searches for quantities. A little is nice but too much can make your soap rancid.
Coffee! - it gets an exclamation point because I like it so much. You can throw some coffee grounds in your lye water and let the heat of the lye reaction do the work for you. Stir to break up any coffee ground clumps and add to the soap. It will make the soap brown and scrubby. GREAT kitchen soap!