TSandM:I haven't done anything about any of the "milestone" dives since 100, but I'm thinking I ought to do something to mark 1000, which is coming up soon.
You shouldn't do anything other than invite your friends. Your friends should take care of the risk. My friends presented me with a bag full of gifts (most of them gag gifts) on my 1000th including a bottle they filled with water from that dive and an engraved flask filled with brandy.