A teaspoon of raspberry freezer jam really packs a lot of flavor. It makes it taste like the stir in sweetened kind.
I also like a tiny bit of honey, with cocoa powder... makes it taste like chocolate pudding. Delicious used as a dip with fresh strawberries.
Peaches with nutmeg and cinnamon really gives a peach pie kind of flavor, too.
If you haven't guessed, I eat a lot of fat free plain greek yogurt;-)