I've always liked using worksheet-style programs. Excel is the best-known one, but Mathcad is what I'm really talking about. They're document-oriented, yet interactive. The emphasis is clearly on making something a human can read; but it's also executable.
I think a worksheet-style programming environment would be interesting. For one thing, it would make it really, really easy to accumulate test cases. I think most developers do informal testing of the variety "I just finished coding X; let's run it and see if it works." If the interactive environment in which you did this kind of testing were a worksheet, you could very easily save the thing and run it again later. It would almost be hard not to build up test cases.