I wrote earlier about using yellow LEDs to light structures internally. Having just started construction on a new module, I've decided to apply this technique, and have sorted out some better specifications than the ones I previously posted.
The advantage of LEDs is that they are dim enough that, with proper positioning, they won't make the structure walls "glow" in the dark, even if you don't paint the insides of the structures black (a chore I despise, and one which presents many opportunities for accidentally painting over places that need to be glued). Their dimness also means you pretty much need one LED per window, sometimes two for a big window.
So, because even a simple structure usually requires several LEDs, I've been constructing circuits of 6 LEDs. I bend the leads of 6 LEDs, solder them positive-to-negative in series, then solder a 220-ohm resistor to the positive end (a value that will give the right current to the LEDs using most transformer accessory power), and two wires to the opposite ends, making a kind of circle I call a "rosette." This rosette/array of LEDs can then be arranged inside the structure, so the LEDs are a millimeter or two behind each window, pointing right at it ... of course it's a good idea to diffuse the light by backing the window with vellum, frosted mylar, paper, etc.
Each of these circuits will consume 20-25ma, depending on transformer voltage. I use a single full-wave bridge rectifier to feed the circuits (since the LEDs only conduct in one direction). As you can surmise, you can power a large number of LEDs using only a fraction of the power from one transformer.