A great website to use to add on to the science lessons that will already be taught. Maybe use these at the end of the day when, or even let kids explore some science websites on their own during projects.
This is a list of 100 social studies apps/websites/resources for kids. If one of them pertains to a specific unit/lesson you can have the students play/explore the content. You could also have them play these social studies games in their free time.