I was going to say berries, too - I use wild blueberries or mixed berries with cherries. It's my staple. (Sometimes I add nuts instead but that adds a lot of calories.) I have also used protein powder - chocolate is a good one but I have no idea what brand it was now. Again, adds about 100 calories but vastly increases protein content, if you ever need that boost.
I use steel-cut oats; I cook up a batch and then refrigerate or freeze in individual portions for heating later.
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