Uphill will get you more muscle and good thing about it is that also the bum, arm, back & stomach muscles get workout when you are going on a hard uphill. Don't worry, you can't get really bulky by cycling, but you will get lean good looking muscle. Only the swimming & rollerblades give you better buttocks.
If you want ideas for cycling training, attend a spinning class a few times. It's a lot harder than actual biking, but I have gotten into idea of doing interval training when biking and also altering pedalling standing up as fast as I can just to get heartrate up when there's not enough hills.