Twitter feed from a site I like to visit, FreeTech4Teachers and there was mention of a site for virtual tours, but I didn't really like any of them. But a lot of places now have them. For example here is one of Mt. Vernon and here is one for the Grand Canyon and here is one for the US Congress. Think of a place you might want to take your students and then Google "virtual tour + the place" and see what you come up with for them.