I have (and will) post some extra “challenge” problems from time to time. Let me explain their purpose so that you can work these problems into your schedule in an appropriate way.
Problems that provide more challenge can be found in two places: the Classes page (listed next to each Day# as “Challenge“) and the Conferences page. The former mostly involve analysis of data generated from molecular models. The latter are taken from a set of once-a-week night-time conferences that I offered back in 2013.
The “challenge” problems on the Classes page provide an introduction to analyzing model-based data. They also demonstrate important points regarding organic chemistry. I strongly recommend that you at least read the summary points attached to the answer keys (these are embedded in pop-up notes scattered throughout the answer key). However, the “challenge” problems generally do not represent the kind of problems that you will find on quizzes so don’t spend time working on these unless that time is available.
The “conference” problems are more on a par with the Sapling and textbook problems. I wrote these problems with the general expectation that a “conference” problem could keep a group of 3-4 students working (thinking-discussing-drawing-correcting) for at least 5-10 minutes and maybe longer. This means that some of the conference problems are more difficult than what will appear on quizzes, but you might look into them anyway as a way of checking the depth of your understanding.
Bottom-line: the place to start your practice work is with Sapling and the problems in your textbook. These provide rich and varied sources of problems to practice on, and they probably come much closer to the level of difficulty found on quizzes (you can verify this for yourself by looking at old quizzes + answer keys).