For a more UI / Web based slant, I would recommend these (beyond a Spreadsheet - which is incredibly helpful for understanding data flow systems):

* Simple video game using Unity or Unreal (understanding perf limitations in a game where 30-60 fps is critical helps make performant interfaces elsewhere - even on Web)

* A simple Javascript framework similar to React (again, will help understand data flow & handling events)

* An http library wrapper around XMLHTTPRequest (fetch exists now, but understanding how to send & read HTTP requests from scratch will help in debugging any problems down the line like CORS issues, OPTIONS requests, etc.).