Hi there! I am just back from the World, and I'll tell ya what I did....
I am a big water drinker, and used to regularly spend $10 a day AT LEAST just on water, because the flavor of what comes out of the fountains just didn't work for me.
This trip, I vowed to save that money, and took my trusty SIGG bottle (one of the aluminum ones, with the "bottle" top, not the twist off") and bought some different single-packs of sugar free tea/koolaid/crystal light. It fit perfectly into my Heys crossbody bag, which is small, but has the mesh pouch for a drink.
It worked fabulously. I'd get water out of the fountain, and add my mix (i liked the green tea with honey & lemon the best --- I will NEVER take any that have color (grape, punch, whatever) because those little powder packs can make a holy mess... it takes so little of the powder to make a massive stain... learn from me). I didn't buy a single bottle of water the whole trip. I'd rinse out the bottle at night, and use a washcloth to clean the top.
Also! We stayed at POFQ, so we had fridges... I took a Brita pitcher and filled it every day so I'd have water in the room at night. That also worked really well.
Good luck!