Scholastic's Computer Lab Favorites provides an engaging way for teachers to introduce new concepts or reinforce skills. Select either K-2 or 3-5 grade level groups and then either look at all the subject areas or select a specific content area to see what's available. There are many fun, learning activities for many different subject areas.
In addition to the activities for the students, Scholastic provides lesson plans and a list of resources to help teachers. It's well worth taking time to check out this site. Then when you have more time, check out all the other resources that Scholastic provides for teachers.