I read their staple diet in the desert is creosote bush - particularly their yellow flowers. I got some yellow mini carnations, and I wash them really well, and pull the petals out and put it in their salad.
They seem to love it. Oh, I found out that carnations are apparently safe for iggys.
If you haven't tried it, give it a shot - mine love it!
Igs I've had have liked them, too. Zair likes them still. Nice addition when they're fresh. Keep in mind that if you get them from the grocery store or nursery there are chemicals in the flowers that could harm the igs. Do not get pre-cut (any florist kind) and if purchasing from a nursery allow up to 3 months before the residue is worked out of the inside of the plants.
Every now and then, I buy the organic flowers they sell at Ralph's. It comes in a little plastic container in the produce section and isn't too expensive. I figure that if it is safe for human consumption, then its ok for my iguana. Adds a nice touch of color and variety.