It's totally up to you but BP's tend not to be the best show snakes. Your BP just wants to find a nice secure place to hide out all day, It's not going to care about a climbing tree or multiple levels or a pool with undergravel filter and drain system, in fact that pool sounds like it would be a huge pain cleaning with a big potential for a bacteria breeding ground.

Again if you want to make an awesome looking terrarium go for it, is does sounds like an awesome idea for some other reptiles, but just remember whatever you build will be for you, your BP will not appreciate these little decorative things.

Just make sure none of the "features" will adversely affect your BP. Remember BP's are VERY security minded animals, the enclosure should be overall secure feeling (at least 3 sides covered up, not much exposed) with a cool side and a warm side with a 88-92 degree in hide warm spot. Having Identical hides would be best , otherwise your BP will pick the most secure feeling hide as its favorite and stay there, even if the temperature becomes uncomfortable for it.