Jack wants to start a vlog on motocycles. He bought a GoPro Session for it, but he can't record audio to it. So he's looking for an external recorder that he can sync to. Leo says his smartphone will work for that. He could get the Rode Reporter and Rode Rec app. All he'll need at that point is to get an external mic with adapter for the phone.
Another app option is Auphonic, which will upload to the cloud and he can edit in the cloud as well. He can upload 2 hours of audio every month for free.
Another option is the Zoom portable recorder. The Zoom H1 is a great entry level handy recorder and it comes with a line in.