Small/silly things I did early in my career that are milestones in retrospect:
1. Compiling my Linux kernel - still relevant 25 years later
2. A multi-threaded socket server
3. Implementing each of the OO design patterns
Other things on-the-job that developers do early in their careers that teach a lot but foist technical debt on to others:
1. Automating any thing that you are asked to do manually (be careful human factors-wise)
2. Picking an overwrought solution to a problem (e.g. template metaprogramming or some darling language)
3. Unit tests for your code in whatever framework that suits. Even just a single test for sanity check purposes
Things that I wish some new hires would do:
1. RTFM (particularly, know their way around API documentation: usage, return codes, errors, exceptions)
2. When advised to RTFM actually do it