I've heard of others planting their floor of their enclosure, but there's a couple of things to consider:
1. Drainage - how are you going to drain all your excess water so that you don't end up with a giant mud-puddle?
2. While the pooh as fertilizer works outdoors, it's not a good idea in a confined space. We try to remove the pooh as soon as possible from the cages, because it breeds bacteria, and if your cham were to go exploring and accidentally ingest it, or even just walk through it it could be bad news.
3. If this is your first cham may I suggest to not try anything too fancy and instead focus on perfect husbandry. Then, down the lines you can start experimenting with grass, or backgrounds after you have the basics down pat. Just my 2 cents.