Although I don't use it myself, I hear many great things about the Wavebird wireless controller for the GC. A friend of mine has it, and when he drops by to play, he always brings it along. I've used it myself briefly, and haven't noticed any differences from the wired one as far as functionality goes. But then of course you have to replace batteries, although I don't know how often.

Or you could always order an extension cable.