I found some really useful workbooks at Sam's club the other day.  All different types, subjects, and grade levels available.  They begin with Pre-K and go at least through 6th grade.  Price is between $5-$8 each, and all are very thick and full of activities.  I am finding, also, that the activities in each book vary enough to keep interest, whereas in other books we've used it's the same thing over and over.  There were topics I hadn't seen before for the younger group, like "Time & Money".  It is worth it to flip through and make sure the book matches your child's level, as I found the grade association not to match our current level.