Ooh, get 'The Unicorn and the Wasp'! It is delightfully ridiculous. And 'Turn Left' is sort of amazing, but you'll need to watch the rest of the series for it to really make sense, so perhaps that's not the best suggestion.

Moffat wrote the two-parter 'Silence in the Library' and 'Forest of the Dead', so you may want to check those out, but I personally wasn't a great fan of them. It may just be that I'm not a big Moffat person, though (I liked 'Blink' and 'The Eleventh Hour' very much, but that's it), and I don't think Doctor Who tends to pace its two-parters very well (which may be the reason I don't consider myself a big Moffat person; four of his episodes are halves of two-parters).

I am really, really excited that you're watching Donna!