Write a compiler. It gives you great insights into parsing/tokenizing, building syntax trees, implementing syntax and Symantec checks and code generation. Obvio

Write a compiler. It gives you great insights into parsing/tokenizing, building syntax trees, implementing syntax and Symantec checks and code generation. Obviously, you would have to pick a toy language (typically a subset of a known programming language) and your generated code will be inefficient. Still, you would learn a lot.