There is no problem getting your drink on in Texas, though alcohol is not sold or allowed inside PAX itself.
The River Walk has plenty of options within easy walking distance of each other, and plenty more choices around San Antonio. The only downside is the locals tend to call it a night early. Some streets away from the River Walk were eerily quiet by 11:00 PM.
Be aware that the legal drinking age is 21 and you’ll need to have a photo ID. And please, drink responsibility and stay safe.
Each PAX has a pre-PAX bar crawl put together by the community. If you visit the official PAX South forums you’ll find a thread for the A Shot of Ice and Fire bar crawl. It is both a great way to tour the local bars and meet people before PAX starts on Friday.