If you'd like to add some variety and adventure to your next 4x4 run, consider Geocaching along the trail. Geocaches are placed along trails to encourage others to visit the area, and often direct the searcher to something of interest that they might not have otherwise known about.
For those unfamiliar with Geocaching, it's an activity that uses a Global Positioning System (GPS) receiver and other navigational techniques to hide and seek containers, called "geocaches" or "caches," anywhere in the world. A smartphone can also be used as a search tool, and there are Geocaching apps available for both iOS and Android devices.
A typical Geocache is a small waterproof container containing a logbook where the geocacher enters the date they found it and signs it with their established code name. After signing the log, the cache is placed back exactly where the person found it. Larger containers such as plastic storage containers (Tupperware or similar) or ammo boxes can also contain items for trading, usually toys or trinkets of little financial value.
Geocaching.com has the largest listing of Geocaches and a Basic membership is free. A Premium membership is available for a fee and it provides a number of upgraded benefits. This site has a very comprehensive "Geocaching 101" page that should answer any questions you might have. There are literally hundreds of caches (and thousands in larger cities) in any urban area and many can be found along 4x4 trails.
The next time you head out on a run (or any other time) don't forget to load the nearby Geocaches into your GPS (cell reception can be spotty) and get some "finds" along the trail. You can also put together your own container ahead of time and take it with you. If you find a good place to hide it during your travels you can create your own Geocache for others to find.
Handheld GPS devices are recommended for Geocaching, and many are made specifically for the task. They range in price from $100 to $600, depending on the features desired. There are also many vehicle mounting options for windshields, rollbars and handlebars.