You may want to consider the Sturmey Archer S2. It is a 2 speed that is shifted by backward rotation of your crank. I have on on a bike that previously was a single speed. What you get, depending on how you set it up) is either a normal gear with a high for down hills or a normal gear with a low for hill. The advantage is that it requires no cables. The downside is that it takes a while to get used to the shift angle and it's very easy to find yourself in the wrong gear at a stop (especially if you have opted for a normal - high setup). You kind of learn to avoid though. A 3 speed hub is not that much heavier and with the additional gear and cable shifting, is perhaps a more flexible option. I do like the simplicity of the 2 speed, though.