If you really wanted a grass look,I'd go for indoor/outdoor carpeting,or astroturf!You could paint your floor and seal it,maybe a bathmat in the same color?
How about something like the head of a chinese dragon-I think a water dragon is called a Lung dragon?-over/around your shower head,or faucett,so it looks like the water is flowing from its mouth.If your mirror is on the wall designated for water,paint a few tiny ripples,and maybe a tiny leaping fish onto it in one corner.If it's on the earth wall,flowers or leaves around it.
I could see using 4 different dragons for the four elements.On your earth wall-hanging planters with vines.Plants like the extra humidity,just find one that can tolerate low light.Maybe phothos or ivy? If you only have room for one or two pots,go with a vine that can trail/be pinned all over the wall eventually.You could mosaic their pots to go with your wall.Candle sconces on your fire wall? If you're worried about burning actual candles,use led light candles.Mosaic holders/little shelves onto your walls.You could do the ceiling as sky and the floor as earth,if you want to use two walls each for fire and water to make a bigger impact.Put glow in the dark stars on the sky,or use glow in the dark paint.Have the sky shade from day to night.Three dimensional clouds.Birds in flight.What is your bathroom sink like-a pedestal sink,part of a vanity,or just the sink attached to the wall? Could you decorate the support like a tree trunk/stump,and then the sink itself would be a pool supported by it?If,for example,you use tiny red tiles for fire,and tiny blue tiles for water,on ajoining walls-have them scattered into each other at the corner,instead of a strict demarcation.
Yes,I know I'm getting carried away.