Stocking stuffers: Mask defogger, small Ikelight, Knife that hooks to your BC, spare mask or fin straps, Scuba Multi-tool, spare o-rings, dive books (Last Dive, Shadow Divers, and HellDiver's Rodeo are my favorites).
Big Gift (probably in this order): Wet Suit, Regs, Dive Computer, Tank, Dry Suit, Camera...got the last two on this list from Santa.
Since most non-divers don't feel qualified to buy scuba equipment, it helps to specify EXACTLY what you want or hook them up with a person at your LDS that can help them. Last year, for example, my wife knew I wanted a camera and strobe. She worked with the LDS -- they added the critical knowledge and even sneaked it into the back of my car while I was on a dive. I ended up driving my own gift home from the LDS without ever knowing it. What a great wife!