Was Silvanus Thompsons lovely "[Calculus made easy][1]" mentioned already? It's a classic (100 years old) freely available on [gutenberg.com][2]. Some opinions of it can be found on [mathoverflow][3]. 

It doesn't go very far so it might need to be supplemented with another text, but I believe it does a great job at teaching the physical and geometrical intuition on differentials. It seems that it's closer to synthetic differential calculus than to non-standard analysis in the way it treats infinitesimals.


  [1]: http://en.wikipedia.org/wiki/Calculus_Made_Easy
  [2]: http://www.gutenberg.org/ebooks/33283
  [3]: https://mathoverflow.net/questions/58193/leibnizian-calculus-textbook/58196#58196