We have 2 windows that are at ground level - people walk by all the time and can see in, especially at night. We live in a really dark area (Alaska) so I wanted something that would let the maximum amount of light in. I bought Gila Privacy Control window film and installed it on half the window (the half that slides open). It was pretty easy - just follow the instructions closely.
I then put some of those cool wall decals in a botanical theme from Joann's over that. They have lots of different ones. If you live in an apartment, the whole film can be peeled off when you move out. I don't think you could use the film again, but the decals may be ok to re-use.
On the other half of the window, I hung a tension rod with a lightweight curtain out of a cotton fabric. We can slide the curtain over to see out the window, and have to remove the tension rod to open the window (though not very often... it's cold here!). As an added bonus, the tension rod acts as an additional lock for the window (think of the piece of wood many people put in the track so no one can force the window open from the outside.)
At night, when the lights are on, from outside you can see the outline of the botanical stickers.
I may go back to Joann's and get another decal to use on this next window... they had a small tree with birds.
I hope my instructions made sense!