Satsumas/tangelos/clementines/etc. Why did I stop eating these things things in college? High-school-me was completely right to bring them to school every day. They're cheap and healthy and dessert-level delicious, and they don't upset my stomach the way normal-sized oranges do. They also for some reason complement both iced barley tea and hot green tea really well, which is of vital importance if you are me.
And they're more convenient to carry around for hot-weather hydration purposes than a nalgene of water. They're essentially juice wrapped up in a little fiber, and that fiber's both really lightweight and unlikely to leak all over the contents of my purse. I have a big handkerchief that's now designated specifically for wrapping up mandarins in my purse; some mandarins and a couple bananas are now my hanging-out-someplace-hot-for-a-while kit.
Tangentially-Related: I got a big thing of soba, but it's too hot for making pasta; I wish that refrigerating soba didn't turn it all sticky and weird. Maybe I should just find some buckwheat groats and try them in the rice cooker, then dump hontsuyu over them? Would that be weird?