The best geocaching containers are those that blend in with the environments in which they're hidden and can stand up to the outdoor elements. Geocaching containers need to be safe for geocachers and animals in the area, so plastic and metal containers, which are non-breakable and do not have any sharp edges or corners, work well.
Military surplus ammo boxes are some of the best geocaching containers. They are durable, large enough to hide lots of geocaching treasure and they blend in with the environment. The way the lids latch on these containers ensures a dry environment for the contents. The only thing is they're bulky, so have to be hidden well to avoid accidental discover by non-geocachers.
Altoid tins work well as geocaching containers since they have easy-open lids, are small enough to hide well, but big enough to contain small items. When using these tins as caches, however, it would be a good idea to place the geocache log book inside a small plastic bag to make sure it stays dry.
Plastic peanut butter or mayonnaise jars are nice options for inexpensive geocache containers. The lids screw on, providing a watertight seal for the contents and the size is perfect for hiding geocaching swag.
Film canisters often make for devious geocaching containers. They are small enough that they can be really tricky to find in outdoor environments, but they're large enough to hold a paper geocache log book.
A flashlight without its batteries would be a great geocache, but you'd have to hide it really well so someone wouldn't pick it up and take it or toss it, thinking it's garbage. You can store the geocache log and "treasure" in the empty battery compartment.
Any watertight, non-breakable containers would work well for geocaches. You may need to choose your geocache hiding spot before you decide what kind of container would work best.
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