Always handy to have some skies handy...shoot a few whem you are out and about.
You could colour the sky as suggested then copy the layer and set the blend mode to multyply or colour burn...if it is too much lower the opacity of that layers...it's fun.
 Get a moon or something from somwhere...crop it out and do the same thing as suggested above and also try what I suggested for the sky.
Also with the sky you could do a Hue/Saturation adjustment and change the hue slider....weird but more fun.
Eddie