I would buy an airship yacht and race The Supreme Canuck.

Fund a drilling submarine mission to Europa.

Get the finest film-makers in the world to do a moon hoax just to show how hard it is to do as convincingly as the real thing.

Buy a Wii, a cactus and some new shoes. (What's wrong with being easily pleased?)

Get the finest film-makers in the world to film some SF books properly - e.g. the Harlan Ellison version of Asimov's I, Robot rather than the filth that we got.

Play a really expensive practical joke on an old school friend of mine that's only marginally funnier than a practical joke that would have cost less than £3.80.

Some of the worthwhile things that people have already mentioned.

__________________
Nothing beautiful was ever made from gravel.