an actually difficult question! Different persons will absorb different things from articles, and will enjoy different projects as a first encounter. Pointers:

https://lispcookbook.github.io/cl-cookbook/ (and see the Emacs or the debugging pages to see what's possible)

see https://www.youtube.com/@CBaggers/playlists and either his introductions to Slime, either his introductions to CEPL to play with graphics interactively,

also for graphics, a new 3D system in development: https://www.youtube.com/watch?v=liaLgaTOpYE

for an overview of how thought through is REPL driven development in CL: https://mikelevins.github.io/posts/2020-12-18-repl-driven/

and, we are lucky (or cursed :] ), there are many more cool articles on the topic.