3) Put it in flight mode and use an external GPS
Will only work if you have a wired external GPS. But most of the external GPSs I've seen communicate via either Bluetooth or Wifi. Which will be turned of in flight mode.
For the record, I have a simple suction mount that I stick onto the windscreen and put my iPhone in. Turn on SD and off I go. No ill effects whatsoever, apart from the light interference on the intercom. A full charge will give me about 1.5 hours of flying time without having to faff about with chargers and such. If the flight is going to be any longer I charge the phone via the cigarette lighter, if available, and otherwise I've got one of those 5000 mAh power packs.
With the iPhone on the windscreen I've never had GPS reception problems.