In rough order of least to most self-absorbed:
1) Charity work -- libraries, the cure for everything, pretty much the whole schmeer.
2) Subsidization and promotion of science education. Too many people in this nation believe that science is frightening or somehow immoral, and that mortifies me. Biology is your friend, people. DNA will not eat you. This would probably also get into heavy subsidization of genetic engineering research, AI research, and such.
3) Paying for public buildings and art fixtures, starting with significant donations to my college to de-uglify the campus. In the era of cookie-cutter houses and Office Depots, we need some beautiful public architecture. If they could do it in the Bronze Age and the Dark Age, why can't we do it now?
4) A family foundation; this may go under charity work, but overall I think it's a noble idea. (My mother's family actually established one after my grandfather's death, in his name, to give scholarships to students from his hometown heading to university; who could argue with that?)
5) If I never have to work again, it's time to learn how to do something useful. I'd try to gain some useful and interesting skills, from brushing up on my high-school German to finally taking Tai Chi. I would almost certainly have a substantial wood shop installed on my rambling palatial estate and get down to making furniture.
Of course, if I end up actually doing research science for a living, even with gigantic scads of money, I'd probably keep working. I'd just have to buy...
6) ... a working Jacob's Ladder and workbenches filled with conical flasks of mysteriously-colored bubbling liquids. And possibly a disembodied brain in a jar to advise me on my evil plots.