Silk has a Dark Chocolate Almond Milk that is perfect for things like this and it's low in calories. You can use that or your favorite mocha mix, soy milk, etc. I find this particular product to be perfect as it's really chocolatey, not too sweet and a good consistency.
First pick your favorite K-cup pack and set your Keurig to iced-setting or espresso setting (you want lower water/oz. as you will brew over ice) and fill a tall plastic tumbler or tall travel mug with ice and brew.
Once it's done brewing within seconds/min. add the Silk Dark Chocolate Almond Milk almost filling it, but topping off with more ice. You can seal/shake it up if you have a tight lid/cover; add a straw and you're good-to-go! Quick & easy. Enjoy!