So I like the idea of introducing a Will-o' Wisp, much like the dragonfly/silkmoth/ladybug in that it wonders around and you can right click to catch it, that would be both a curio and a possible light source that you could put into something to glow.
You could either have them appear only at night, only in caves, or following the actual legend, make them show up in bogs and swamps along with dragonflies. The actual design of the creature would be a simple orb looking thing so it would be simple to create. And above all else, it fits into the motif of the entire game.
Pulling slightly from this thread: viewtopic.php?f=5&t=1205&p=14033&hilit=will+o+wisp#p14033