Apparently there’s a hidden easter-egg in the latest version of Google Maps for iOS, that allows us to cache maps for offline viewing. This is what you need to do: just line up your map to the area you want and type ok maps. That’s it. If you’re in a supported area, the maps will be saved. If not, you get an error message telling you the the area you’re on isn’t covered. This happens to me in Barcelona, so you might consider avoiding it on your vacations.