I was gonna say strawberry rhubarb, but Soliloqy beat me to it.
Seriously. You should try it. My mouth is watering just thinking about it.
I usually just throw some frozen strawberries in my KT glass as "ice", and as they defrost, they flavor the drink. Easy and yummy. I've been doing continuous brew for awhile now, so I don't do a second ferment (I don't want to bother with extra jars to clean). But the frozen fruit in the cup is great. I keep lots of frozen fruit on hand for this purpose...blueberries, rhubarb, raspberries, blackberries...