This is not strictly (purely?) category theory, but two fun little projects that will teach you a lot (or reinforce what you already know) are writing your own

This is not strictly (purely?) category theory, but two fun little projects that will teach you a lot (or reinforce what you already know) are writing your own parser combinator library (https://www.cs.nott.ac.uk/~pszgmh/pearl.pdf https://www.cs.nott.ac.uk/~pszgmh/pearl.pdf) and Lens library (https://en.m.wikibooks.org/wiki/Haskell/Lenses_and_functional_references https://en.m.wikibooks.org/wiki/Haskell/Lenses_and_functiona...).