KScience's site features a broad selection of interactive animations covering different scientific subjects, including a detailed look at how the kidney works. Students can mouse over the different parts of the kidney for a brief summary of their function, or choose from a drop-down box to take a closer look and the workings of each element of the kidney. Among other activities, kids can choose substances that pass through the nephron and view the paths they take, or enter a sodium level and see how it affects the loop of Henle.
The website Biology Corner has a page explaining in even more detail how the kidney works within the excretory system, and has a detailed diagram of the kidney to color. You can print out copies of the page and distribute them to students, together with the included coloring directions. The page explains what color each component of the kidney is supposed to be, allowing kids to visualize what it looks like as they learn about how it works.
After teaching kids about the rest of the excretory system in conjunction with the kidneys, have them try Purpose Games' quiz. This game presents a diagram of the system and displays the name of an organ or blood vessel. Students must then click on the part of the excretory system that they think the name corresponds too. The quiz keeps track of the mistakes you make, and gives encouraging messages when you make the right choice. Sound effects add to the fun, and upon completion you can add your name to the high score board, which tallies your time and percentage rate.
As a final activity, have the kids put together a small model of the excretory system using everyday objects, as suggested by the website Teachnet. Print out or draw an outline of the human body, and then glue kidney beans into place for the kidneys. Glue short lengths of red and blue yarn for the veins and arteries, unraveling them where the blood vessels branch out. Break off segments of spaghetti and stick them into a small marshmallow for the ureters leading to the bladder. Cut a short length of drinking straw and insert it into the bottom of the marshmallow for the urethra. Glue the whole thing into place.