Any caffeinated drink will help you feel alert and motivated. Coffee or caffeinated sodas are my personal favorites. If you can stand the flavor, drink the coffee black to prevent sluggishness from the sugar and dairy products. (For that reason I prefer coffee over sodas.)
Depending on your metabolism and individual reaction, you may need to manage the amount and timing of your caffeine consumption, since consuming too much and/or too late in the day may affect your long-term productivity and ability to sleep. Also, my experience has been that I am more productive if I skip the caffeine on less-demanding, vacation-like days. Our bodies develop a tolerance for stimulants, and continual heavy use may lead to severely decreases effectiveness. Switching up the type of stimulant might have the same effect, and may work better for you, since I am guessing that you never slow down long enough to have less-demanding days!