What is the best substrate for an iguana?

I agree with the first answer. Astroturf would probably be the best choice. I used it with my ball python and it was by far the easiest to clean and maintain, it also makes the cage look nice as well. The only problem I had was that it did tend to get a little moldy from time to time but I was having to keep the humidity at a very high level. The alfalfa sounds like it might be a good choice to go on top of the astroturf, IMO.