Several people have given some great, easy ideas for flavoring. Remember to never add the flavoring before culturing - only add after growing the ProBiotics and cooling (e.g. artificial sweeteners can inhibit the ProBiotic growth or starting with chocolate milk was gross).
Crystal Light: Add to a finished bottle of fresh probiotics to sweeten and flavor
Stevia & Lemon Extract: One customer adds to ProBiotics made from whole milk (Adkins or Keto Diet?). He says it tastes like lemon meringue pie.
Flavor Packets: Some have used Kool Aid or other flavor packets for a quick way to flavor an entire bottle along with the sweetener amount and type of choice.
Fresh or Frozen Fruit: Blend up the fruit and desired sweetener and add to an entire bottle (e.g. bunch of fresh strawberries with 1.5 cups sugar for fresh strawberry shake - best flavoring ever and very beautiful coloring!). One customer blends in his frozen peaches that he grew last year. At the dollar store I recently saw mixed berries and fruit to flavor an entire gallon for just $1!
Freeze It: Some put into ice-cube trays and then use that in place of ice to make green and regular smoothies. It gives smoothies some protein and flavoring to make more satisfying and not as watery.
Chocolate Protein Powder: One customer adds chocolate protein powder to finished shake to boost the protein and add sweetness and chocolate flavor.
Flavored Syrups: The kids practically inhaled the Cherry syrup version (Cherry flavoring like used to make cherry 7UP). There are a lot of other syrups you can try (e.g. Hazelnut sounds really good).
Experiment: Half the fun is trying new things. Maybe pour individual bowls or glasses of unflavored ProBiotic yogurt and then experiment with a number of different flavors or combinations. (I still want to try and make a coconut cream pie version.)
Let us know what favorites you come up with and post them on FaceBook or email them to info@EZYogurtMaker.com