Use every nav aid including GPS, hell if you get the time put the test route into the GPS as well. If the ATO does not want you not to use it then they will turn them off (simulated failure), but if you demonstrate your ability to use everything the aircraft has on board it only proves your proficiency. Also use the auto pilot if you have one. This probably goes against the grain a bit from your training as your instructor might give you the impression its cheating, but its not. If it was a real charter after all, its exactly what you would do, use all the tools at your disposal! And most of all don't be afraid to make decisions, doing nothing is the worst thing you can do.
Good luck!