Whether or not you grade formative assessments, they really aren't about the grade. Formative assessments provide students with detailed feedback about how they're doing, and how to do better. They also provide you, the professor, with a sense of how well your class is doing, and where you might want to change things for all, or provide special assistance to those who are struggling.
For example Classroom Assessment Techniques, or CATs, are small, formative assessments that don't burden class members (including you) excessively, while providing everyone (including you) with helpful data. As the name suggests, they are traditionally done in a face-to-face classroom setting. But why not adapt some for an online course?
Many of these overlap with, or are the same as the active learning exercises described in Week 3. That's by design!
Consider the following CAT possibilities. Depending on whether you want students to collaborate on them or not, you could use dropboxes, discussion topics, or some other web-based communication with these.