Terra cotta flower pots make nice hides - with such a large cage many hides would be good. I prefer "azalea" flower pots because they are shorter & wider than normal flower pots. I use either a wood rasp or a hole saw (fits into a power drill) that is meant for cutting ceramic to widen the drainage hole in the bottom of the flower pot, and also cut a semi-circular hole in the rim of the pot for a "door." I stand the flower pot upside-down in the cage, so it is like a nice big cavern for the snake. The terra cotta is great for slowly releasing humidity over time if you soak it or even just mist it. That may not be the look you are going for, but my BP's love them!
Also, I have a plastic vine in with my female (came with her and the cage from her first owner) that she loves. I never would have bought the thing myself, but she loves to crawl through it or hide in it. Tangled somewhere in that vine is also her favorite place to poop - so you want to make sure that if you get any decorative vines they are easy to clean!
Something that is slightly more decorative is Spanish moss. I buy large packages of it at a home improvement/gardening store for cheap - but it is a good idea to put the package in the freezer for a week or so before you use it, in case there are any bugs hiding in the moss. It gets a little messy because bits and pieces break off, but it isn't bad. I use Spanish moss about 2-4" deep on top of newspaper for my Dumeril's Boa, and she seems pretty happy in that. The moss is also fair at holding humidity for a slow release.
Good luck, hope that helps. 
~Rebecca
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1.1 Ball Pythons (1.0 '05 Ghost, 0.1 '03 Normal)
0.1 Dumeril's Boa '04
0.2 American Pit Bull Terriers (40 lb darling lap dogs)