Just write in the letter what you hope the doc saw in you....and what you thought you displayed during your time with him.
For example, if you did some reading about some stuff related to his practice or specialty..."Jo-Schmo Student displayed a genuine interest in my practice and remained inquisitive throughout. He/She illustrated a natural ability to seek answers to the questions that arose as he witnessed clinical medicine."
If you worked your ass off..."(name) proved to be an extremely motivated observer and participant in my clinic. He/She often saw opportunities to help the staff, as well as myself, with both clinical and clerical duties - which was very much appreciated during busy times. In addition, (name) took advantage of educational opportunities in the office by spending a vast amount of time talking with patients, observing nursing duties (injections, plebotomy, etc). "
If you've volunteered and stuff....(NAME) and I discussed at length his/her experiences with the HIV clinic and his travels to South America during volunteer medical mission trips. I was quite impressed by his/her genuine desire to make medicine available to all parts of the world"....
whatever....
Just think about what you tried to portray during your time with the doc and write it down. Us the thesaurus for descriptive words...