I would pay off my student loans (wouldn't that be great!).
Then I'd get a real house, one which I could have roommates if I wanted (though preferably a family) or not if I didn't, so I wouldn't have to climb to my 4th floor walkup. And I'd stock that house, and hire a cleaning service so I never had to wash a dish or dust a cabinet again. It would have a state of the art darkroom so I could finally get back into all stages of my photography, as well as super-great computer systems in which I could do digital work as well.
I agree with Entropy on the library thing, and mine would include a climate-controlled vault for rare and historical documents, mainly family history documents and photos.
I'd expand my antique camera collection.
I'd travel to every place my ancestors ever lived, and live there for 6 months to a year at a time, writing and researching and just getting to know the people of the area. That would include Scotland, even though I've already been there, and England and Ireland as well. Also, Sweden, Germany, and Poland or Lithuania--it would depend on where I finally pinned that East Prussian ancestor down to.
I'd never work again, and spend all my time researching my family history or writing, whether or not I ever got published.
Then after I visited countries to which I have a personal connection, I'd make a world tour to visit all my international roommates: Korea, Brazil, Laos, Belgium, Canada, and Mexico. Then I'd start visiting international friends from China, Taiwan, Japan, Australia, Ghana, South Africa, Thailand, and Russia. Then I'd start on all the countries I've found interesting but never had a connection to: Italy and France (great art), Mongolia, Tibet, Israel, etc.
I'd get allergy shots so I could own horses and all sorts of other furry animals.
I'd buy my dad and stepmom a new home and a cleaning service, as well as a complete wellness package, including help on quitting smoking. Included with that would be people to repair things when they broke (not 5 years after, or in some cases never, such as how the bathroom sink hasn't worked since I graduated from high school 12 years ago), and also supply them with all the farm needed to run successfully. I'd also supply them with lots of land to go trail riding on.
I'd buy my mom her own place with resources for all the things she never does anymore--like her art, her sewing, that sort of thing--because she can't afford it or feels that it makes her sick (she's mentally ill). Along with that I'd bring in the best mental health experts to help her be able to live a full life again.
I'd give my brothers enough money so they could be financially stable for the first time in their lives, then as a stipulation for that, have them finally go to college and learn life management skills and family conflict resolution skills.
I'd repair my grandma's home so that she'd be able to live there comfortably without having to worry about the state of the roof or whether the furnace will work this winter.
And of course I'd dote on my sister's son and the new little one that's due this week, as well as be able to visit my other nieces (my brothers' daughters) and have them come visit me.
Mainly, the money would free me up to do things I want to do now, but don't have the money to do.