Chapparal Ice is nearby....
Austin's Park and Pizza is there and a lot of teens hang out there. I'm not sure about a birthday party though. Depending on how old you're turning. If you're over 11, you should just hang out with some friends there.
Tinnseltown movie theater is there also.
The malls are a bit farther though and they're not in the Pflugerville city limits.
Good luck ;)
I can't remember what all is in Pflugerville, but isn't Austin Park and Pizza close by? It has (at the time that I last went) go-carts, arcade games, mini golf, bumper boats, laser tag, rock climbing, and probably some other things, too. I believe it's on I-35 around the Grand Ave Parkway exit.