You can use 'Mode > Grayscale' but you will not have as much control as using Channel Mixer. Another way is to use Gradient Map. Both of these methods will definitely yield better results.
Okay about masking. Here is how you do it -
Using Channel Mixer/Gradient Map:
Go to Image Adjustment Layer > (Channel Mixer > adjust the contrast and tone)/(Image Adjustment Layer > Gradient Map > Grayscale mapping select white to black) > OK. Next to the Channel Mixer layer you will see a Layer Mask > choose a black medium soft brush to paint over the areas that you want to be in color.
Using 'Convert to Grayscale'/Gradient Map:
Command-J (Mac)/Ctrl-J (PC) to duplicate a layer > Mode > Grayscale > Click the New Layer Mask button in the Layers palette to create a mask that reveals the selection > choose a black medium soft brush to paint over the areas that you want to be in color.