I like Celestia. You can do some really simple but effective things with it.
One of my favourite demos is just looking down on the north pole and speeding time up. A very nice day/night explanation that complements the usual globe and OHP thing.
Even better, if you speed it up further (I think another 100 times??) you get to see the terminator, the line between day and night, oscillating back and forth across the pole, which describes the origin of the seasons lovely.