Students in Grade 4 and above are often beyond the "toy" programs of learning their letters, numbers, playing hangman and the like. It can be hard to think of fun educational activities for them to do with computers. Below are some ideas that will appeal to many students well into secondary school.
Provide students with access to a few webcams and let them learn how to manage images on the computer. Introduce students to the joys of Gimp where they can do such things as photo retouching, image composition and image authoring.
Allow each student to write emails and send pictures to a penpal in another country.
Create a MOO (an object oriented multi user game) for students to play around in and practice their creative writing skills while learning basic programming concepts. See these for more information:
Introduce them to Squeakland which is a media authoring tool to create their own media or share and play with others.
Introduce students to programming by showing them cingb a Gameboy-emulator for all kinds of platforms.
Let students have fun with graphical teleconferences using drawboard. When they draw something on their table, all remote users can see it. It features a client-server architecture and a high-color palette.