Firstly, I'd make sure that the money wouldn't disappear. To acheive this i'd start up my own company that will eventually churn out more cash (ala microsoft??), then i'd expand this company to scientific research in many areas. Especially in the fields of space (materials, propulsion, colonisation) and robotics.
On robotics, I would make robots which help in emergencies and detect and record crime activities. If they could be made self sustaining and repairing I would distribute thousands of these bots around the world to countries who wanted them. This in an example: A pedestrian hit and run has just occured. A robot would detect the disturbance immediatly and notify other robots. Some would chase and photograph the car whilst others attend to the injured.
On space, someone else said it before, the future is out there. I'd hope to catalyse the indutrialisation of space.
Another thing i'd do is employ a team of planners. Someone here said fusion would save the world...but, would we use our vast energy wisely? We'd probably all waste it and thus heat up the earth. The planners would plan for that.
Finally, make sure we can't destroy ourselves with war by developing strong defense weapons to disable any nukes flying over the earth etc. I would not give these weapons to anyone but myself
All this whilst giving to charities.