The Science Sense Teacher is happy to offer a download that includes 12 cards that offer practice in identifying independent and dependent variables. These cards can be used to enrich, remediate, or just get additional practice. While this product is intended to be a colorful, interactive bulletin board, it could also be used as task cards or any other practice method you'd like to use. There is a chart to record answers and reasonings.
I like to have one required activity that all students participate in every day and then offer time for "choice pieces" which allows the students to decide which activities they will engage in to enrich their knowledge of the subject. I often use an interactive bulletin board as one option for the students.
Blackline master copies are available. Please just ask!