Canvas has a number of ways to manage extra credit (and for an explanation, see How do I give extra credit in a course? . This document will elaborate on one of the options discussed briefly in the Canvas Guide, namely, using weighted assignment groups to grant extra credit.
My approach to offering extra credit in my fully-online classes is to allow students to augment their grades by demonstrating robust participation in ungraded discussion topics (the assignment is pictured at the end of this document). On the first day of classes, in the syllabus, and through course announcements, I advise students that they can earn the extra credit over the course of the semester, and that they will be able to see their contributions reflected in their grades after the last day of classes passes.
So that Canvas can factor the grade for the extra credit assignment into the final grade automatically, I use a weighted assignment group scheme in which the required assignments in the course add up to 100%. I then create an additional assignment group, worth 0%, called "Extra Credit: Participation," and I place a single assignment in that assignment group. You can make the assignment itself worth any number of points that makes sense for the requirements of the assignment and the grading rubric you plan to use. I mute the assignment so that I can grade it behind the scenes.
After the last day of classes, and before I have to submit my students' final grades, I grade the extra credit assignment, making sure to assign a grade of zero to any student who did not complete the activity. At that point, I change the weighting on the extra credit assignment group from 0% to 1.4%. The 1.4% grade contribution is my personal preference; you can use any extra credit percentage with which you're comfortable, as long as it's greater than 0%. Lastly, I unmute the extra credit assignment. Students will now see how their extra credit contributions have impact their final grade.
It's best to use this approach only if you're willing to wait to grade until after all of the other assignments have closed before granting the extra credit. If you don't, and if the extra credit assignment group contributes anything other than 0% to the final grade in the meantime, students will see a somewhat confusing reflection of their cumulative course grade. James Jones explains this better than I ever could:
I think the most useful solution I've found...is to make the extra credit worth 0% until the end of the semester. This is especially important when you have a category like "final exam" that's worth 30% of your grade. Until your assignment groups that actually have grades in them exceed 100%, Canvas will scale the grades. So, if you have homework 20%, exams 50%, final exam 30%, and extra credit 10%, until the score is entered for the final exam, you have 20%+50%+10% or 80% of the grade. Since that's less than 100%, what you're calling "extra credit" isn't considered extra credit by Canvas (you can't have extra credit if you don't have more than 100%) and gives a wrong (in your thinking) grade.
If someone has 90% on homework, 60% on exams, and 100% of extra credit, then they have 90*20%+60*50%+100*10%=18+30+10=58% out of a possible 80% which gives them 72.5% for the class. What you want them to see is 90*20%+60*50%=18+30=48% out of 70%, which is 68.6% for the class, or maybe you want 68.6%+10% extra credit = 78.6%. I'm not sure, but you probably don't want the 72.5%
Now, when the final exam grade is added (let's say they got a 42% on it), you have 90*20%+60*50%+42*30%+100*10% = 18+30+12.6+10 = 70.6% for the class. Since there is now 110% possible, which is more than 100%, Canvas doesn't scale the result to make it worth 100% and so the extra credit works the way people expect.
If you want to allow your students to choose from multiple extra credit opportunities, you can put multiple options within the Extra Credit assignment group and adding grading rules to the group. If you do this, you will need to assign grades to every student, including zero grades for any students who did not complete the extra credit.
Or, you can use separate Extra Credit assignment groups for each opportunity.
For those of you who are interested in seeing the actual assignment I use to measure course-long participation, I'm providing a screenshot below. James's excellent resource--How to Count Student Discussion Posts--is invaluable in this regard, in that before he devised this solution, I had to do manual searches by student name and tabulations on scratch paper. James's solution is considerably easier to implement than it looks on first blush.
Sample Extra Credit Assignment for Course Participation
(Last, it's worthwhile to mention here that Garth has devised a method of overriding a grade with the end result of lowering it; his goal is the opposite of awarding extra credit, but he uses the same methodology as what I've described above: Override / Adjust the Final Grade)