Bags come in all shapes and sizes, colors, fabrics, and prices. The only thing that matters is the functionality of it. Maybe you choose a bag that has a way to tote your yoga mat around or has extra pockets for accessories (maybe rings/jewelry, wallets, cellphone and hair ties). If you are a minimalist, consider a simple, no-fills bag with ease of access and simplicity in mind.
So what do you have in your bag? There is no need to stress about having everything—just think about the essentials! A good place to start is a change of clothing. Unpredictable weather and that “pop-up” class may spring up at any time; and a change of clothes to keep you dry and comfortable should you need to make a stop somewhere after class—is a must.
Hydration is important! Pop in a water bottle ready to fill. There is nothing worse than getting a tickle in your throat mid-pose and not having something to soothe it—or getting into your hot yoga class and needing to be quenched. This leads right into bringing a towel or two. Needing to wipe sweat off your face or hands and not having a towel can be uncomfortable; and if you have long hair—don't forget your headbands or hair ties. As you further ponder how to pack your bag, you may think of nice-to-have (but not absolutely necessary) odds and ends to tuck away; such as an extra phone charger and maybe your favorite mat cleaner.
One more last thing—after a session your bag may have accumulated packed clothes, towels, your water bottle and maybe a few yoga props; don't forget to unpack your bag along with your best self in the four corners of your mat and honor the time you have taken for yourself!