Try to write a pure POSIX applications on either iOS or Android.
https://www.usenix.org/publications/login/fall2016/atlidakis
https://www.usenix.org/publications/login/fall2016/atlidakis
It's a pity the Ubuntu phone went nowhere. There is Ubuntu Touch though (haven't used it).
So I rephrase, try to write a iOS or Android app using UNIX only APIs.