I actually have ended up thinking about my marketing efforts and watching videos about that instead of tinkering with the app (or other appealing ideas that inevitably emerge during down time).
Do you have early customers/friendly users? Have you done any UI testing? Paper prototypes? Do you have a clear idea of your users' goals?
While dated, read "The Design of Everyday Things" and "The Inmates Are Running the Asylum".
If you don't have users even to the level of being able to provide early feedback for non-functional UI designs, then you are probably doing too much coding and not enough user testing.
(That's easy for me to say, but there's a decent chance I'm wrong.)