Check out "Google AppSheets". I've only scratched the surface while investigating other stuff but it's basically "drag-and-drop mobile GUI builder w/ sheets as a backend". If it were 2005 it would SLAY so much code. As it is, it seems really useful but the outcome seems a bit generic for modern tastes.
This seems quite hidden. Googling "google appsheets" results in "did you mean 'google sheets'?" and showing only results for google sheets unless you specifically then request AppSheets.
It sounds cool, but I'd hate to rely on it since Google will probably shut it down.