Cross Out god.
Its simple, it's easy, and it's not intent on making currency unusable, so it's not technically illegal. I've seen plenty of dollars with lots of stuff written on 'em anyhow, and I've used the penny-press machines at more than one national monument.
It's not like our dollars are issued by the government anyhow, they're issued by the Federal Reserve. Even without the "in god we trust" it isn't honest money, they're a rip-off. But that's not an atheist issue so much as it's an American problem.