Quick Tip: Carry a reusable water bottle around instead of using disposable cups or plastic bottles (especially PET ones).
You have many options and, much like the cloth bag, I highly recommend you get a bottle you really love and want to carry around.
There are 4 different options (or if you want to think of them as such, schools of thought):
1. Reusable plastic bottles: many people believe that regardless of the type of plastic, some of it may still leach into your water and consequently, your body. There are different types of plastic and this option is better than one-time use bottles but as far as production and disposal goes, they're still produced from fossil fuels and can only be downcycled.
2. Aluminum: is light and quite cheap. I have an aluminum bottle but I'm not in love with it. I do, however, still carry it around everywhere. It's lighter than glass, which is convenient but I'm not sure how long it'll last me. The problem with aluminium is that mold tends to grow in the bottles eventually (though this can be cleaned), so to prevent this, I empty mine and leave it to dry every night.
Not the most beautiful bottle (Asaklitt) but for 5 euros from Clas Ohlson, it's treated me well this summer.
3. Stainless steel: has all the perks (light weight) of an aluminium bottle with the added advantage of higher quality (for which you pay the higher price). If you're looking for an entirely plastic-free stainless steel water bottle, Kleen Kanteen has launched one. (Featured on myplasticfreelife.com) Or check out the product on their website here.
4. Glass: There are some really nice glass bottles you can get, for example, this glass one with a bamboo sleeve from the Canadian site Life without plastic. The bottles are a bit heavier but also very versatile. One of the best suggestions I've read for reusable drinking options is made of glass: a mason jar. Since glass is really easy to clean, you can use it for more than just water (coffee, tea, hot chocolate, juice, soup...anything!) I've actually been having some issues finding mason jars in Finland but since it's packing season now at the end of summer, you may have luck checking out what the bigger grocery stores have in stock.
Quick facts about water bottles here. (I don't completely agree with the 'solution' part of that link but the problem and impact sections are without bias.)
If you want more information on this stuff, do a Google search to see which choice is best for you and what products are offered in your area (or if you're within shipping distance of some of these online stores.)
If you have more thoughts on the subject, feel free to comment and start a discussion.
N.B. After posting this, I found this from a trusted website:
"Plastic may leach chemicals into the water and aluminum bottles are lined with an epoxy resin, some of which has also found to leach into water depending on the brand. Why take a chance? Read [Beth Terry's] posts about bottled water for more information:" http://myplasticfreelife.com/category/issues/issues-bottled-water/