Get your son to make her breakfast in bed, even if you need to do it and he supplies the tray.
After that make a point of spending the day with her, it doesn't necessarily matter what as long as it's something she likes, just focus on you and her hanging out. You know her far better than we do, despite claiming she has no hobbies she clearly has interests. Go to the spa with her, take the hit and make her laugh by getting waxed or whatever the hell they do to people in those places. Look at local theatres, there's bound to be a few that you don't know about on your doorstep.
I'm about to hit 40 myself on the 20th of this month and don't want to do anything either, but when I say that I mean I don't want a fuss made. I want to actually enjoy the day rather than be forced into some draining family event, the only thing I want is a nice quiet day doing things I enjoy with the people I love.
I can't speak for your wife, whether she's some sort of super social party animal or not, but from my own perspective if someone threw me a massive party I'd hate it and feel as though the day became about an event/everyone else and it'd be exhausting. But for the love of god, whatever you do, don't look at it from a "value" perspective.