Constructing an adventure from a mad idea
Al Preuss whom some of you will know as deafnala on various forums very generously sent me some of his amazingly characterful hand crafted toadstool folk. Obviously these needed to be used in a game, my first thought was to have them as 'sentient scenery'. Base them up and leave them in the corners of locations so the players could interact with them. Ideally they could confer useful local rumours or background information. Unfortunately I already have Gossip Stones for that and anyway, these models needed more of a starring role. 
What could I do with evil toadstools? Of course, Mario!

​The idea of dropping the player characters currently adventuring in my on-going fantasy role playing campaign into a  Mario world where the dangers and challenges were real appealed to me. I'd already used elements from Zelda games in my setting, this was just taking it one step further. I had an adventure lined up involving a pocket universe created by a wizard. I hadn't begun to finalise the world inside the pocket universe but now I had my starting point. 

The plot
This was obvious. There's a bunch of innocent mushroom people who are now in danger because their princess who used to protect them has disappeared. The standard Mario plot.
The player characters are on a quest to break the time tombs of the Izveroni before the Golems of Meaning arrive on Yarth to do it themselves and destroy the planet in the process. I gave the princess one of the hidden keys to a tomb and added her into the helpful instructions they were given by the spirit of the long-dead Arch Mage Afgorkon. 

"The key to another tomb is held by Princess Persikka* of the Fungal Folk. Delve deep into the caves beneath the forests of the Peaceful Sleepers to find her. These forests lie beyond space and time in a pocket universe created by the wizard Thaladomis."

*Persikka is Finnish for Peach. I didn't want to make it too obvious. 

The build up to finding their way into the Mushroom Kingdom was standard stuff. There's a very powerful magician, a very powerful emperor, a couple of very powerful demons, a horrible curse and an immortal gryphon. And then, after all that, there was a clearing in the forest of the Peaceful Sleepers with a green pipe in the centre. 

The Mushroom Kingdom
What was in store for them? Well, to be honest, far more than I'd originally intended. Once I started compiling ideas for the adventure more and more stuff was just too good to leave out. 

The opening location
The Town of the mushroom folk. The players would meet the Mushroom folk, find out their situation, learn that the princess was missing and, by way of setting the tone, fight off a raiding party of Goombas. 
I had my Goombas, they had started this whole thing. I needed some Mushroom folk. I was going to buy some but the ones I found were all expensive and quite warlike. I settled on 3D printing some I got off Thingiverse designed by Whystler.
I decided to use some existing huts I had for their homes. The last thing I needed were some travel pipes. I printed these as well. 

Continuing the story
Operating on the principle of 'more is better' I decided to add as much Mario flavour as I could to the mix.
The adventure would require the players to visit several locations to collect Stars in order to open the final gate to the level where the princess was being held.
I designed Boo's Mansion, Monster Mountain, Yoshi's Island, Kirby's Prison and the final Platform Peril levels, populating them with all kinds of Mario enemies and a few friends and allies.

Boos Mansion
The first setting I finalised was the haunted house where the Boos live. I'd found some Halloween ghosts on Thingiverse that would make excellent Boo stand-ins. I could have made real Boos but, like the Goombas, I wanted it to be clear what the creatures the heroes were meeting were meant to represent so the players would recognise them but dial everything down a few notches to be more in keeping with the 'reality' of the characters' world. 
I admit there's not much 'real' about my Boos but their deadliness as opponents would counteract their silly appearance soon enough. 

​I already had a suitable model for a haunted house and some graveyard sections to add to the rear. I populated the graveyard with some printed floating pumpkin Jack o' lanterns. I added my printed Demon Braziers to the mix because they seemed to fit the theme and consulted my Mario 64 game for other creatures. 

​The Mario 64 game had an angry giant piano, no problem- I printed one, and Big Mr I. I printed a giant eyeball for him and added a wire and hot glue optic nerve as a support. While I was at it I printed some smaller ones so he'd have minions. With some comedic but deadly traps (scratch built with 3D printed elements) the deadly haunted house was complete. 

​As a final touch I added in Toad, the hero of the mushroom people who'd tried (and failed) to find the missing princess. Toad could accompany the party and offer helpful insights into the world they were exploring. I printed Toad as well. Rescuing Toad and defeating Big Mr. I got the players their first Star. 

Yoshi's Island
The next location was Yoshi's Island. I decided to split this into several separate areas so I could introduce more Mario styled elements a few at a time.
Yoshi's Island introduced Koopas (in several forms), Piranha plants and Bullet Bills. All these ended up being 3D printed.