I got started programming doing similar things; hacking copy-protection code on games on my 48k spectrum, so that I could go on to patch the games for infinite lives:

https://blog.steve.fi/how_i_started_programming.html

Later I started doing similar things on shareware and trial-locked applications for the PC, via sites such as +fravia's reverse-engineering site.

These days people put out "crackmes" which are fun challenges if you want to test your reverse-engineering skills, and while I always pay for software these days, when I need it, there's still a lot of fun to be had patching binaries to allow your preferred serial number to be accepted!