My first thought was "turning 40 next week and you're still in the ideas stage? You're a dead man!"
But if low key is what she wants, you're ok!
Nothing to offer, because in our family it's big things for the major birthdays (we went to France for my 30th) but my wife's 30th is coming up in the fall, so I'm well into the planning stage, and am interested to hear the suggestions put out in here!