As a general rule, simplicity in the cage is best. The inside can be rather plain. A rather large hiding space is also a must. This will make them feel safe and secure. This is actually a nice feature of monitors, as long as you are diligent in changing the water.
Some monitors are aborreal and require branches to climb on.
These must be sturdy enough to hold the weight of the monitor, which can be quite heavy in adulthood. Many monitors can be tamed sufficiently to be handled often. To encourage taming, I recommend placing an article of clothing in their hide box. This will help them associate your scent with security and safety. There are a few things to remember though.