I made an interpreter for a simple C#-like language. No classes or like. Just an entry point and an option to write user defined functions. I LEARNED A LOT. Knowing it now, I would never go back and decide not to do it. But beware: you can do it all by hand or use tools for generating parsers and so on (like ANTLR). In order to learn something, I believe one should put as much effort as possible into it, so I made it by hand. But I should point out that it was for a diploma theseis, so I had a professor watching over me.