Depends on what sort of light you want and how you define 'easy'...
LED strips are fairly straightforward plug & play, but they mainly create whole areas of light inside the case.
There are various other add-in mods that are purely decorative and have various numbers of LEDs in various shapes. There's a puck one that shines light in a 360º circle, for example.
Then you could always follow my idea and rip apart something completely unrelated to harvest parts for modding the PC yourself. Just make sure your power supply (usually a 4-pin Molex) supplies the right voltage and that the mod is connected to the appropriate pins.
I actually got some circuit boards made up with LEDs on them, which don't really flood the case with light, but instead shine like the indicators on some motherboards... in fact, that is what the circuit was designed to do anyway, reading values off the mobo and changing the LED patterns based on the activity/speed/status of various components - I can't *just* have pritty pritty lights... well, I can, but it's always better if they actually mean something too!