My daughter is NOT picky eater, but still, from time-to-time my kiddo will decide she doesn't want to eat her normal go to foods. I don't always mind this, it gives me an opportunity to try out new foods on her. I do have an issue with it when i have 7 bananas left or a full box of oatmeal wasting away.
To fight her food rejections I got creative. I dressed up the snacks. For a banana I slice it in half, add a little peanut butter for glue, and then I let her decorate the banana with shredded coconut and SPRINKLES! Sprinkles are key! I can let her add them to anything and she'll eat it. Boring oatmeal, add sprinkles and its the best breakfast ever. A baked sweet potato, add sprinkles and its golden. Raisins are good too. My three favorite mix-ins are shredded coconut, sprinkles, and raisins. I like them since I can store them easily once she's over the food blues and then I have them for the next round of, "No don't want that!"
I've been doing this for 6 months now and it never fails. Let your kid get creative (even if it grosses you out) and they'll eat foods they think are tired and mundane.