Be prepared for heat!
Austin is in South Texas which means the summers are dry and hot whilst the winters are- at coldest- mild. In summer, temperatures can remain around 30°C all day with clear blue skies. As idyllic as that sounds, it also means you must remember to bring with you sufficient water for any outing you plan (or at least know where you can get a refill) and be applying (and reapplying) sun cream all day! Also, make sure you have snacks on you as you burn more calories in heat than you would think.
Remember: nothing ruins a holiday like sunburn and heatstroke!