Cucumbers and green peppers do it for me. I also try to keep my broccoli intake to a minimum. But as often as I tell myself it's not going to be pretty, I can't stay away from the Chinese buffets.
The tank overflow tube? Stick a stopper in it for a couple overnighters. And don't light a match. Do you have a particularly warm garage? Do you park it with a full tank or does it have a lot of room for vapors to rise in it. I believe the California models have a special filter to eliminate or minimize the vapors from escaping.