Seems lame, but my advice is have fun. Do something you'd never do in a longterm campaign. I like running the occasional 1-shot and I use it as an opportunity to imdulge my players. I'm a fairly conservative DM, so in the 1 shots I tend to let them go crazy: 3rd party sourcebooks, all evil party, inter character conflict, whatever.

If you want to run an open session just let your players sandbox. If you want to actually tell a story its okay to pregen, just make sure your players know there is a plot based reason for. Then make sure each character has a role: the traitor, the secret prince, what have you...