Not a hypothetical guide — these are the actual trips I took, with the actual restaurants that were safe, the actual trains I caught, and nothing left as an open question.
Tokyo, bookended — Hakone, Kyoto, and Osaka (with a Nara day trip) in between, entirely by JR train and Shinkansen.
City StopSplit across the start and end of the trip — a gluten-free omakase lunch worth booking twice, and buckwheat crêpes that lived up to the hype.
City StopA cycling tour through Arashiyama, a traditional tea ceremony, a Kyoto Sanga match, and the soy milk ramen spot found by accident.
Glasgow to Fort William to the Isle of Skye to Inverness to Edinburgh, entirely by train, ferry, and bus — with a celiac-safe restaurant pick for every stop.
City StopArthur's Seat, Edinburgh Castle, and three of the best celiac-safe dinners of the whole trip — including a 10-course tasting menu at LYLA.