Should you do ALL, or SOME, or NONE of the Extra Credit? No matter what, be intentional and plan out what you can reasonably accomplish. Make the assignments count. Really "watch" the videos or "take interest" in the supplemental events, lectures, or activities. Don't just skate through these, or skip over sections, or misrepresent your work. Don't cheat yourself out of the value of the activity (not to mention that this would be a violation of our codes of academic conduct).

Extra credit is optional. You do not need to do it to be successful in this course. However, extra credit activities can help boost your grade in several ways. You get points and you also spend more time working with the material. However, students sometimes have trouble assessing the impact of extra credit and deciding if they should do it and how much of it. I discuss here a few scenarios.

Students should consider time management issues and whether they have enough time to do the extra credit so that it is not at the expense of regular work for the class. Classes are designed so that you get the most return on your investment of resources. In other words, the teacher has thought about what is the most important material for you to review in order to best prepare you for the quizzes and tests and for success in the course. The extra credit may be valuable and interesting, but it has been assigned as extra credit because it's not critical to the main objectives of the class and/or it supplements material that has already been presented. Do it if you can, but not as a substitute or replacement for regular course material.
Sometimes teachers will provide extra credit for supplemental activities (such as an outside lecture, solar observation, or star party). Although the activity itself is not a part of the course and your success in the course will not depend on your participation, the experience can be thought of as an opportunity (like a famous speaker), or can help engage you more fully with the material or course (like a club solar observation or star party), which can greatly benefit you (personally, professionally, or academically). Extra credit is assigned to encourage you to participate and to reward you. It is important, though, to go to these events ONLY IF YOU WANT TO GO! Do not go just for the extra credit points, but go because you wanted to go anyways, and the extra credit serves only to reward you for doing what you already wanted to do! Going for just the points is the wrong reason to participate and defeats the purpose and intent of the assignments. Go because you want to go and have fun with your fellow students because you all want to be there!
It's important that students don't feel "obligated" to do the extra credit. It is what it says it is, "extra"! As teachers, we want to provide a deep and rich introduction to the material and there are many amazing resources we want to show you. By presenting some of these resources as extra credit, we can allow you to explore on your own a bit, but you should not feel pressured to do it all. In fact, students who put themselves under that type of pressure can burn out and feel overwhelmed. Don't do the extra credit unless it's something you want to do and you have time do it.
Some students feel they should do ALL of of the extra credit and sometimes will work HARDER for a few points of extra credit at the expense of regular course work. It is important that students prioritize the regular coursework first. This material has been designed to best prepare you for success in the course. The extra credit is "extra" because although it's helpful, it may not be as beneficial a return on your time investment as compared to the regular course material. Always do your regular assignments first before spending a lot of extra time on supplemental extra credit assignments.
On the other hand, some students underestimate the beneficial impact of doing at least some extra credit. Although the point values may be low, they are pure points on the top, like frosting on a cake. While a regular assignment may be "out of 10" or "out of 100", extra credit is always "out of zero". For example, if you do several assignments (let's say 3 assignments) and get 7 out of 10, you are averaging 70%. Now if you do one 10 point extra credit (so same point value as the 3 other assignments), and you get 9 points of extra credit (ok... you missed one), that means that you can take three of those assignments that you averaged 7 out of 10 points on, and effectively turn them in 100% assignments (the 9 pts can be thought of as 3 sets of 3 pts that boost each of your three regular assignments 7+3=10). So, just a few extra credit points can be applied to multiple assignments essentially boosting your grade significantly even though the point value of an extra credit assignment may seem low. So, an extra credit assignment that is about the same points as a regular assignment can go a long way to help boost your score on multiple assignments (in this example boosting your score on three assignments from a C to 100%).

You should note that although the extra credit example above makes a big difference (boosting three assignment scores even though the point value of the extra credit and that of the individual assignments were similar), note that if you had missed (or skipped) one of the required assignments (because let's say you were focusing more on the extra credit), then you would have lost 10 points off your final grade. So the penalty of missing one regular assignment exceeds all the value of your extra credit. Now you are doing even worse even with the extra credit, than if you had just done the required assignments with no extra credit added on top.

In other words, it's ALWAYS best to do your regular work first (do all required assignments) before doing extra credit, then do what extra credit time allows and/or if you are interested in some of the supplemental topics covered. A healthy balance is to do some of the extra credit, but don't try to do it all!