In my water dragon viv i have got:
Loads of plastic floor plants
vines and leaves hanging from side of viv for climbing and hiding in
big tub of water to the side
a fair few branches for them to climb around
lilly pad floating in their water (pointless but they seem to like sitting on it)
Thats what ive decorated mine with and mine are always running round and happy. Personally as stated before as long as there is something for them to climb on and a hiding spot (using plants) and a feeding area its normally fine. Ive got vine type plants going on both sides and back of viv mainly because i noticed they sleep in them so i covered the viv in them.
Big tub of water cos they LOVE swimming hence the name.
Waterfall because ive noticed mine drink from that and not the tub (mainly cos they poo etc. in the tub)
fake plants to recreate the natural enviorment and makes it lok bette in their to.
Hope this helps