Lights
One thing that spices up night scenes is adding lights- street lamps, stop lights, crossing signals and billboards. Wire these all to a common bus, and then to the AC terminals on your powerpack.
Wiring Techniques
There are essentially two different ways to connect your lights to your powerpack: series and parallel. Series circuits keep all lights at the same brightness, but when one burns out, the whole system will not work until that bulb is replaced. If you use parallel circuits, the more lights you add, the dimmer they all will be (this effect is because of an inverse relationship of resistance in parallel). Of course, when one burns out, the rest of the lights will continue to burn. The best method for you depends upon how many lights you have. For most people, using a combination of the two techniques is the easiest- that is, place multiple parallel circuits in series. However, keep the size of those series circuits to a minimum. If you can, run several sets of common bus wires underneath your layout.
Many modelers find it effective to use an older powerpack instead of the AC terminals on their train powerpack. If you do this, you can run your lights off the "track" (DC) connections. Then when you vary the throttle, you're actually varying the power voltage (brightness) given to your lights. Just be careful not to turn the dial all the way up unless you have a significant number of lights in series, or else you could exceed the maximum voltage of your lights and blow one or more of them out.
Constant Intensity Lighting
Have you ever noticed that your locomotive's headlight brightness varies with the speed of your train? Many modelers have found ways to make that light burn constantly, or at least at a constant voltage. Most methods use diodes and/or resistors to step the voltage down or keep it constant. Other modelers insert batteries into their locomotives, but the fit can be tight. A variation on constant intensity lighting is called directional lighting. That is, when you reverse the direction of your train, the lighted end of the locomotive also changes. I don't have any circuit schematics here, since constant intensity lighting deserves a separate discussion. However, I wish to explain the concept. If you are interested in persuing this, check the NMRA's links page for help.