An insulated 64-oz "mug" of Water.
Seriously, the mug is almost as big as my head. It lasts me all day, though, which is why I got it... not to mention, if it was cold when it went in, it'll still be cold HOUUURS later.
When I bought it (gas station market), I filled it up with some soda, no ice. The soda was still cold, 9 hours later, and even still carbonated. <3 my mug.