The one-way mirror would be great, except for the physics of it. They're only one-way when the light on the outside is more intense than the light inside, which kinda makes things awkward at night. You may have seen this effect in the cities, when the one-way mirrored windows of office buildings are suddenly transparent at night.

However, there are these cool films that you can put on your window that will let the light in while making it hard to see through them. There's all sorts of options to choose from, and it's cheap and easy to install yourself. http://www.gilafilms.com/Residential/residential-window-film.htm has more info if you're curious. :D