The best option of available FAs is Ronnie Brewer. If the Spurs can get him on board on the cheap for a year, that is the best possible scenario. I don't like Howard, but if he's healthy (mentally and physically) he is very talented. Outside of that, Barnes or Butler would probably fit the bill. The options go downhill pretty steeply after that.
Tracy McGrady is intriguing, but he is kind of a head, so I think he ends up in L.A. or Miami.
I know everybody here hates RJ, but signing him should be a top priority. He is the best available SF with starter ability and the S&T option is appealing if we can get back TE or talent.
If none of these options work out, the pickings get mighty slim and risky. Players like Joe Alexander, Jonathan Bender, Rodney Carney, Yakhouba Diawara, Stephen Graham and Jarvis Hayes are out there, but the Spurs would be taking a big gamble by taking a chance on them.