Q: Is there an easy way to add bonus points to my Gradebook?
A: Selecting the checkbox Make this an extra credit assignment for a extra credit item will do exactly what you want.
You can find this checkbox on any grade-able item (assignments, assessments, discussions and miscellaneous items). You can then specify a total number possible, but if the user gets zero points then there is no penalty. If they do get points those points get added to their total score for the category.
Let's take a look at an example, say that at the end of a grading period, there were a total of 450 points available for quizzes and tests.
Student A had gotten 369 points total on those quizzes and tests, that would give her:
369/450 = 82% before bonus
Student B had gotten 405 points total on those quizzes and tests, that would give her:
405/450 = 90% before bonus
Now, to look at the bonus:
Student A, in addition to her 369 quiz and test points, earned 9 extra credit points from other activities:
(369+9)/450 = 84%
Student B, in addition to her 405 quiz and test points, earned 3 extra credit points from other activities:
Notice in both cases, the total number of points available remains at 450.