The beverage base packages that you find in MRE's are flavored electrolyte mixes. You can get them in several flavors, and they come in individual packets. I use them on the water that I've run through my Katadyn filter sometimes. Filters and MP1's make the water "safe" to drink, but, the beverage base packs make it actually a lot more acceptable from a taste perspective. I have tried the fruit punch, grape, and lemon lime flavors. I think they also make orange. You can get them at places like:
http://www7.mailordercentral.com/longlifefood/products.asp?dept=1031