I can think of a couple. Write a small toy document database with an integrated ORM. Tables/documents can be modeled as objects and the query API can be built

I can think of a couple. Write a small toy document database with an integrated ORM. Tables/documents can be modeled as objects and the query API can be built using a functional paradigm. Doesn't need to be fancy. Could be a simple key/value store really. Or how about an HTML generation library? Or how about a toy web server? How about a command line argument parser for your favorite language? How about a small library for geometric calculations? There are many languages that have multi-paradigm support. One of my favorite multi-paradigm languages in still in it's growing phase, but it's fun to learn: https://nim-lang.org/ https://nim-lang.org/