Reverse geocaching: my goal

I need a gift. The gift receivers to be are avid geocachers. Checking for new cashes wherever they go. I have a little of experience with AVR’s and Arduinos but I’d like to expand my knowledge and put it to the test in a real-life project. Eventually I’d like to be able to use these newfound skills in my job as an R&D engineer (where most of the programming & electrical engineering stuff is outsourced). Hence my goal of this project is to improve my programming skills, work methodically and document everything and of course end up with a kickass gift.