Never, ever, ever work for a paycheck again. And give my mom and siblings and a few choice friends the same.
Buy a lot of acres ... some mountain, some shore (not necessarily all one location), but ideally driving distance ... build a small house for myself, a big garden with some help to tend it, only occasional maid service (I'd prefer the dust to her frequent company). If my family's interested, they can all have houses on the same acreage.
Get some private cooking classes, voice coach, sewing lessons (or just a personal seamstress, heck).
Oh god, I would just be so happy if I never had to work for a paycheck again. I am SO STRESSED!
No way I'm going to spend 400 million on myself, though -- I think I would endow some aspiring (religious) hermits, but that wouldn't cost much since they're all taking vows of poverty anyway. Find some really promising projects to invest in in places with lots of poverty or post-conflict trauma ... Somalia, Afghanistan, Bangladesh.... Some kind of support for people with mental illness, research for anti-psychotics that don't feel worse than the psychosis itself. Housing and supportive services for chronically homeless people. Support for veterans and their families. Something to support self-governance for indigenous peoples, U.S. Indian tribes or overseas. Home health care and home hospice for people who don't want to go to nursing homes. Subsidize clean & renewable energy research, or subsidize the cost of solar panels e.g., to the end consumer. Organic food gardens at schools. On and on and on, oh, I don't think I'd have trouble finding ways to spend it!