In the wild, ackie monitors are known to shelter under clumps of spinifex grass. You can recreate this effect with live or artificial plants. Plus, plants always make an enclosure prettier!
Artificial plants:
Artificial plants are convenient because they always keep their size and color, are super durable, and never die. However, this convenience comes with a risk: the plastics and dyes that they are made from can release toxins into the air. The effects that this has on reptiles is still fairly unknown, but we do know that reptiles have more sensitive respiratory systems than humans have, and “indoor pollution” from artificial materials in human homes is rapidly becoming a concern for human health.
That said, if you’re dead set on artificial plants, Custom Reptile Habitats has a great selection of artificial plants that are more likely to be safe for use in reptile enclosures, and looks very realistic.
Live plants:
Live plants have many benefits (increased ambient humidity, increase oxygen concentration, bioactive function, etc.), but there’s one big drawback: when restricted to the confines of an enclosure, an ackie monitor will happily uproot/trample any and all live plants in its environment. So don’t be surprised if your attempts to propagate live plants inside your ackie’s enclosure fail.
That being said, if you’re dead set on live plants, don’t let this stop you from trying. Make sure any plants you use are 1) durable, 2) drought-tolerant, 3) heat-tolerant, and 4) non-toxic. The Tortoise Table is a great resource for checking whether a certain plant you’re interested in is safe to use. Succulents and grasses are generally a good starting point. Although ackies don’t eat plants, the bugs that they eat sure do, and a cricket that nibbled on a toxic plant can become toxic in turn!