I know a lot of people don't use artificial sweetners, but for those of us who do, a packet or two of sugar-free jello along with some dried fruit can add a lot of interest to whole-grain muffins, without adding a lot of calories. If you prefer a lighter fruit flavor from just one pack of jello, add a little Stevia in the Raw, since one packet may not be enough sweetner for your taste. My favorite is adding orange jello with craisens and walnuts.
On deck for my next batch is cherry jello and dried cherries. I was also thinking lemon jello has a lot of potential.