Aside from staying home and drinking tap water out of a washable, reusable glass, the best way to go "green" while on the go is to choose your water bottle wisely.  From the lowly single-use-only plastic water bottle you can buy at your local market to the stainless steel and aluminum options, making the right choice is important to both maintaining your health and the health of the environment!

The lowest form of water bottle is the single-use bottle – made from PET (polyethylene terephthalate -- this is the kind that most beverages are packaged in: sodas, waters, etc.)  This kind of plastic has been proven to leach chemicals such as DEHP (Bis(2-ethylhexyl)phthalate) after repeated use and is a known carcinogen.  These bottles can also harbor potentially dangerous bacterial growth inside any cracks and crevices inside the bottle.

Next to the lowly PET bottles used for beverages, probably the most common would be plastic bottles, commonly used in camping and exercising made from thermoplastic polymers that usually go by the name polycarbonate.  Polycarbonates have been proven to leach BPA, a synthetic hormone that can mimic estrogen and cause prostate cancer.  Even scarier is the fact that most baby bottles are made out of polycarbonate, and when you heat up milk in them to feed your baby, the BPA could be leaching in at an even higher rate than normal. 

A much better option than either of the above two would be an aluminum or stainless steel water bottle.  A big manufacturer of aluminum water bottles is Sigg, which claims to make environmentally friendly products.  The exterior is made from recyclable aluminum and a water-based resin lining.

Another option is a stainless steel reusable container.  Kleen Kanteen is a large manufacturer of these type of bottles.  They are made entirely out of stainless steel, which does not leach. 

Ideally, the best way to drink water on the go would be an aluminum or stainless steel water bottle, as they seem to exhibit the least amount of health concerns out of all the choices.  Add in the fact that they can be used over and over again with no degradation and do not need to be recycled each time you use one, then they really are the "green" choice.  And since upwards of 40% of bottled water is actually just tap water in disguise, buying it seems like not only a health risk due to the plastic leaching possibilities, but also a waste of your hard-earned green!