You don't need much for the day. The easiest way to avoid getting exhausted is to reduce the amount of stuff you're lugging around town. To test if you have too much stuff in your bag, use the taco method. Count everything in your bag, and imagine the things you have as tacos. If the number of things is more tacos than you can eat, put some back.
Here in Austin, we have an excellent city bus system. It's an easy way to get around downtown when there's bad traffic or no parking (and believe us, there won't be parking). If you're a student in Austin, you get a half price discount on bus fare when you present your student ID to the driver. (Prices vary between routes. See Capmetro website)
There is also a bicycle rental system downtown called Austin B-Cycle. There are dozens of rental stations throughout the downtown area. Why not travel faster and make good use of Austin's bike lanes while getting exercise? Day passes are $8 and 7-day passes are $25. (Austin B-Cycle)
You never know how many people will be at a show, so don't assume you can show up fifteen minutes early and get in. If there's a show you really want to see, make sure to get there extra early. Shows at small venues like The Mohawk will fill up super fast, so be aware of the venue size. The free shows at Auditorium Shores are worth getting there way early for. If too many people show up to Lady Bird Lake, the entrances will be closed.
Make sure you bring: