Thanks for the kind review Nico! I totally agree about the need for challenges and in fact sketched out about a dozen or so of them in my notebook. But as a solo dev, with the time remaining in the jam running short, I decided to invest in polishing the sandbox experience as much as possible rather than deliver a half baked version of both.
You can find remnants of the code for a scene manager and a challenges mode in the code still as I started working on it before switching back to polishing the sandbox.