This worksheet helps students to realize when it is appropriate to use different methods, mental math or pencil-and-paper. Once students start taking standardized tests and into adult life, they will need to be able to figure out which type problems can easily be calculated mentally. In the standards, it is listed as "use appropriate tools strategically".
This page does not include many actual calculations but does mention division and decimals. It may be appropriate for advanced 3rd graders and up.