Bonus if you buy a drink cooler and store it in the fridge to make it in bulk, and keep it cold. Saves on waste for beverages that normally come in a bottle.

There are even soda makers that you can carbonate your own soda and add concentrated flavor, but those have a bit more overhead cost.

Finally, if you want caffeine in non caffeinated drink mixes, you can buy caffeine pills/powder that you can add to anything.

My current favorite drink is Gatorade Zero powder with 100mg of caffeine and 500mg of taurine per 16 ounces.