I use a GPS (Lowrance I-Finder..cheapo) when we go on our bi-monthly rides. Mine has base maps built in and I can opt to download more detailed maps. You can find mounts for them at
www.gpscity.com or on ebay just search for RAM mounts. Mostly, I use it to find a more direct route back to the house as some of our ride destinations end in the middle of nowhere at some small town lunch stop.
Once you get a GPS, you can have some real fun with them by Geocaching. Check out
www.geocaching.com. It is an addictive sport and my whole family enjoys going on geocaching treks. You would be suprised at how many caches people have hidden in parks, downtown, train stations, flower gardens, landmarks and more. It is lots of fun and gives you a chance to use that GPS!