This yoga mat bag from YogaDirect is cheap and reliable, which are my only criteria for any bag I can't put food in.
It fits a standard size yoga mat, a small towel, and most importantly, a water bottle. Getting the bottle in there can require a bit of maneuvering, but it's one of the distinct advantages it has over most other bags. The strap hangs nicely and there's a small pocket on the inside, in case you needed a small pocket on the inside. At $14, it's one of the less expensive mat bags out there.