Smartphone maps app. All of this worry about no cell service is hyperbole if you have an iPhone or recent Android. Google Maps has an offline maps option for your routes, so if the cellular signal drops the GPS keeps going just fine. You just have to download the map data prior to leaving for your trip when you're in service or on a WiFi connection. No big deal.
In fact, many of the higher quality 3rd party map apps now offer offline maps as well.
The age of free-standing GPS units is coming to an end quickly, and the idea that I have to pay - several hundred dollars sometimes - just to update an offline unit like a Garman is insane. No thanks!