The thing you want to stay away from when looking at the materials you listed are 1) How they react to moisture/humidity and 2) off-gassing.
1) Leather would be a poor addition, like all other organic material its going to get soggy decompose smell bad and feed bacteria.
2) For all the synthetic stuff the thing to be most concerned about is off-gassing which is the continued curing of the material, CO2 comes out of concrete and all sorts of nasty stuff can come out of resins.
Another thing to stay away from is anything you find outside. I have heard of people boiling rocks to death to add to their enclosure which seems acceptable, but don't put anything organic you find outside, inside.
Since you only have one animal there is no need to think outside the box. I agree with you petstore stuff is boring, so go online. There is a ton of stuff out there to choose from and since you don't need anything in bulk the stuff that is made for reptile enclosures is what you should stick with. Unless you want to do a ton of research.
You might want to get to know your snake a bit before getting decorations too. BPs are NOT arboreal they are terrestrial, that being said I have two girls in glass tanks, one is 1100g and doesn't really climb the other is 1500 and I never see here on the ground (I personally think its because she believes food lives in the sky, i feed f/t, and she is trying to find more). So before you buy a jungle gym of mopani, add a little and see if it gets used.
When I had only one BP I tried real plants, don't even bother, they'll get mauled over the first night.
NOW, first things last, being a new owner...
Can you describe your set up? Or even include a picture?
How big is the enclosure?
How are you heating the enclosure? Controlling heat? Measuring heat?
It's sounds like you're already wise to the idea that the water dish is too small, I can't imagine a 2.5 yr old BP submerging itself in a 6x6 but maybe its really deep.
And there is a generally a lot of good information here in the care sheet.
Welcome to the addiction