Sublimation and recrystallization of caffeine
This sounds like a nice project.

For the purification of caffeine, sublimation in vacuum and recrystallization from ethanol are both conceivable.

But first you have to separate it from the other ingredients, such as carbohydrates. 

According to the manufacturer, 100 mL of Red Bull, a popular brand, contain 11 g sugars and 32 mg caffeine.

In order to isolate the caffeine, two methods are possible:


extraction with $\ce{CH2Cl2}$
solid phase extraction on a C18 column


The latter method might be interesting. Imagine that your pour the energy drink through a preconditioned column of this adsorbent. The caffeine will  be retained, whereas everything else passes through.

In a second step, you wash the caffeine off the adsorbent using ethyl acetate. Then, dry the organic solvent with sodium sulfate, filter off, evaporate the ethyl acetate, dry the crude material and check the melting point. If necessary, use any of the above two methods for purification.

If you'd like to read this in more detail, have a look at a procedure provided at Open BU, an open access initiative of the Boston University.



UPDATE

I'd probably use just one of the methods for purification, depending on the equipment available. But if you insist on using both, I'd recrystallize first.