Great thread topic. Now that I've gotten into Morelia I've been more interested in providing enrichment for them.
Basically, your imagination is your only limitation. Just avoid any adhesives like tape, avoid sharp edges and metal wire (often found in fake plants), avoid aquarium decor designed for fish.
With BPs sometimes something like a hide change or decor change can lead to food refusals and such. You won't have that problem with a curious species like a bull.
You can use cardboard boxes, tupperware, take out containers, paper towel tubes, pvc pipe (wide enough to climb in or build a perch type structure), terra cotta, sticks, rocks, etc... Have fun with it.
If you have any doubts as to whether or not it's safe, don't use it. Or at least ask around here first.
Build little rock caves, being careful they can't crush the snake, tunnels, etc...
But have fun with it, mix things up sporadically. Inquisitive snakes definitely appreciate the opportunity for enrichment.