Here's another concealment/dead drop option delivered by Shomer-Tec. They have recreated and improved on the Cold War era Spy Bolt. The hollow spy bolt was used as a dead drop device by the Soviets operating in West Germany. The Soviets would usually stash messages or other items; then screw the bolt into a desolate wooden bridge railing for another agent to retrieve.
Shomer-Tec's offers a larger cavity than the original (.49" cavity). The 3/4" bolt also has an O-ring that will keep your items from being damaged by weather.
The thing I really dig about the bolt is that it has reverse threading. So if someone tries to open it they will actually tighten it, heightening your security.
You could hide your valuables amongst other nuts and bolts and no one would ever know. Yet another way to hide your cash from your wife...