We have great Thermos brand stainless steel water bottles for our kids. The top opens and a straw pops out. Even our youngest (4) can get his open. The bonus is that they stay cold longer then plastic. They're not super cheap, usually about $12-15 at Wal Mart/ Target, but they last SO long because of the stainless steel- we're going on school year number two with ours. I figured $12 was only a few packs of juice boxes - and now I've saved all that waste I had been making with the throw away boxes!
Actually, I bought another set for my older girls this year- stainless steel ones with the screw off top. They're a little harder for the younger kids because of the screw top (if not on right the can spill) and then you have a wide mouth bottle to drink from. I saw the Sigg brand at Target for $24, I got a different brand (but still stainless, not aluminum) at Home Goods (saw them at Marshalls as well) for $6.99!! And they're so pretty - we have one with pink & green flowers and one with a black and white design and my daughters initial.
Here are the Thermos ones. We have a Spiderman one for our boy (here is a link to a Tinkerbell one...)
http://www.walmart.com/catalog/product.do?product_id=10994452
We have this plain one for our older girls because they didn't want characters.
http://www.walmart.com/catalog/product.do?product_id=10669918