At the beginning of the year one of the first things we do in Algebra 2 is go over types of numbers. We classify natural, whole, integers, rational, and irrational numbers. The next day we do a group formative assessment where in a group they take turns organizing different numbers into rational/irrational numbers. Then I give each of the groups a get a sheet where they have to organize a list of numbers into rational/irrational and explain why.
Students did an excellent job discussing the numbers and classifying them.
The Math Assessment Project has other amazing resources like this one at their website.