One of the most popular types of alternative liquids for cold process soap is alcoholic beverages. This includes beer, wine and champagne. These liquids contain sugar, which give the soap a stable and fluffy lather. Alcoholic beverages can also give the soap color and are great from a marketing standpoint.
Soap making is also a fantastic way to reuse excess spent yeast from the brewing process. Yeast is rich in vitamins and minerals that are also good for your skin and provide moisturizing qualities. For more on these properties, you can check out this thread on the soap making forums.