Quickie Math is the title of a folder that I keep in my planning box. (I use my planning box to hold my worksheets for the week.) Each subject has a folder and the box is divided into two sections, one for each child.

On a day like yesterday where I wouldn't be doing any topical teaching in their core subjects, I was able to have Bethany go to the Quickie Math folder (separate from her Math folder in the same box) and pull out a worksheet to complete. These worksheets are usually lessons we may skip because the content is more reviewing previously taught concepts. But we all know review has it's place. A graded math workbook would have the same effect in that it can be pulled out and taken on the go if necessary.
In fact, I recommend some sort of reader or graded topical workbook for most subjects on those days you do need to take learning on the road.