You can hang out there whenever there's not a concert, for free
Want to run the stairs? Sure. Want to have a quiet lunch? Okay. Want to get onstage and play? Sorry, they put a net over the stage to prevent you from doing that. BOOOOOO.
On that token, you can see acts playing there for free, too
If you're willing to get up early, rehearsals are often open to the public the day of the show, so from about 9:30am-12:30pm, you can go and check out soundcheck (unless the act has requested it be closed).
You can bring your own booze in... usually
How can you tell? Take a look at the schedule and see if it's a "Lease" event or not. If it's not, go nuts: coolers, picnics, and booze in its own bottles. If not, you've got to get creative.