If the geocache is bigger than the tiny micro-containers they're filled with random knick-knacks. The usual protocol is to take something out and out something in when you find it. Some people get VERY enthusiastic and have personalised items or stickers or whatever, I take a pocketful of stuff with me in case I want to keep something, otherwise I just swap stuff between caches. When I was in Jersey I actually took took things home (I had taken a TON of stuff to leave in caches) and gave them as gifts to my family, complete with photos of the area I found them and a description of the area and any adventures I had while finding them. I figured that was a much better gift than yet another postcard or keyring. Some of the cooler stuff I've found in geocaches: Yugoslavian money, a complete incense set, a 40 koki-pen and crayon kit (it was in the mega huge cache), key rings of all shapes and colours and sizes - most of which have been moved to other caches, several compasses, photo-frames, Brazilian money...
I guess this could be a new section: fun for the family! I took this photo while hunting on the cliff for a geocache with my Dad. Having geocaches to look for made my trip to visit him awesome, and it gave us something to do together other than the usual sight-seeing! He was amazingly patient with me hunting around, although he also liked to wait until I was in a particularly ungraceful position to take photos... (so is that where I get it from?)
I climbed this tree in the hunt for a cache. It turned out it was in a different tree, across the road... and at about shoulder height!
I took this after climbing the mud bank to find the very tricky ammo box!