Everyone knows that the best way to win a costume contest at Halloween is to make your own gear -- and be good at it. The costume that's custom tailored for you, and detailed by you, is going to be so much more impressive than something that you can buy off the rack for fifty bucks. Careful selection of appropriate accessories, whether you made them or not, is one way to take your look the extra mile.
Instructables user Honus has gone more like the extra marathon, with his article detailing How to make a Green Lantern ring -- including a glowing version. You'll learn to make several different styles of the Green Lantern's ring, by creating a "wax pattern" (an exact model of the item you want to make, only in wax), a rubber mold, and finally, a resin ring, with optional instructions for including an LED so the ring can glow.
It's a revision of an article that was initially about silver-casting, and those instructions are included, as well. (The author explains that he rewrote the article because most people don't have access to silver-casting equipment, whereas it's pretty easy to cast resin at home. I'd like to see a version of this article written for metal clay, which is somewhere in between in terms of difficulty: it shrinks when it's fired.) Overall, not only will you learn to make an extremely cool ring, you'll also learn a lot about the basics of jewelry design... and maybe come one giant leap closer to the costume contest prize.