[seanpreston]

The closest thing to this I can think of is an offline TripAdvisor app (TripAdvisor City Guides). The app downloads city data straight from TripAdvisor to use on your phone when your internet / wifi is switched off.

Their huge database of places & reviews is combined with a section on 'walking tours' which seems very similar to what you're describing. A great bonus with this is that GPS services still work without internet.

A great addition to this would be bespoke intelligently generated tours based upon your likes or interests.

Also a limitation of the TripAdvisor app is the number of cities offered (though this seems to be increasing)