It kind of depends on why the professor is having you do this. Do you 1) have a great relationship with him/her outside of class and you are basically getting a blank check? Or 2) do you go to a huge university and only know this professor minimally? 2) was true for me, and I suspect it's true for a lot of people.
I go to a huge university and my professors don't really know me, at least in many science classes. I gave my favorites and the ones who seemed more connected to the college samples of my writing--my personal statement, why I was interested in dentistry, some scholarship essays I had written, information about my extracurriculars and general interests, and then I sat down and just had a few conversations with them even though I didn't have a class with them that semester. That made it very easy for them to write the LOR because they had something to say about me.