I've done a ton of
I've done a ton of Geocaching; most of the items inside aren't much more than a happy meal toy, a little green army man or other plastic knicknack (for example little toys found in those gumball machine thingys), marbles, pins, trading cards etc. Occasionally there will be travel bugs (see the link posted above for a good description) Geocoins (similar to travel bugs, not meant to keep but to move on to other caches), Pathtags (similar to geocoins, but meant to keep and track).
Depending on the size of the cache, there may not be room for any goodies at all. The size of the cache can vary a lot, so take the Geocacing.com size description into consideration when deciding which caches to look for. Smaller caches are harder to find (and harder to hold goodies if you're looking for trading items). Small caches are generally no larger than a peanut butter jar; Micro about the size of a 35mm film canister or a magnetic key holder, nano - even smaller than that! I remember finding one that was literally a small ziploc bag that was covered in camoflauged duct tape. Inside was just a few sheets of paper to sign - it was wedged between the rocks of a stone wall. No room at all for trading items (I think it was classified as a "micro" size).
Pay attention to the difficulty levels as well - some caches are very well camoflauged! It can be lots of fun to discover that birdhouse is really a geocache, or that pinecone on the tree isn't really a pinecone after all (or that fake rock, or tree stump, etc.). 90% of the time you're just looking for camo'ed tupperware or an ammo can but every once in a while someone gets creative with the caches they hide (I think these are the most fun and challenging), and it's not always obvious by the cache description exactly what you're looking for, so the surprise is that much more fun too!
Something fun to do is to load your GPS with as many geocaches as possible in the area you'll be staying in, then when you find yourself with spare time, turn it on and go find whatever is closest to you. If you have an iPhone or Android, there's several Geocaching Apps available that will allow you to download the cache descriptions, hints, waypoints, etc on the fly as well (if your phone has gps it can also point you to the waypoint but it will eat the battery super fast.). Some apps are free and some are paid for but they all basically do the same thing.