Smithsonian National Air and Space Museum does offer thirteen good online exhibits. I won't summarize all of them here, but I would like to point out the ones that I like the most.
America by Air online exhibit. American by Air is a series of thirteen online activities that take students through the history of commercial aviation in the United States.
How Things Fly features an interactive module in which students design their own airplanes. The activity starts with a simple and slow airplane that students have to modify until it reaches a target speed and altitude. As students modify the wings, fuselage, and engines of their airplanes they are given instant feedback on the effects of those modifications. In some cases the feedback includes the airplane crashing and the students having to start over again.
At first glance The Wright Brothers - The Invention of the Aerial Age looks like it's just a timeline of developments made by the Wright Brothers. Dig into the Interactive Experiments section of the exhibition and you'll find Engineering the Wright Way. Engineering the Wright Way offers interactive simulations in which students learn about wing design by joining the Wright Brothers for test flights in Kitty Hawk, North Carolina. Two more simulations about thrust and plane control will be released later this year.
Apollo to the Moon lacks the interactive simulations of the three exhibits featured above. That deficiency is made up for by the depth of the content in the exhibit. Apollo to the Moon contains seven chapters chronicling NASA's effort to put a man on the moon. The exhibit begins with a history of the Space Race and Kennedy's proclamation that the United States would put a man on the moon by the end of the 1960's. From there the exhibit moves into the design of rockets and other equipment to put a man on the moon. It concludes with a gallery of artifacts related to the Apollo 11 mission.