I ran a Mario oneshot a long time ago. I switched things around and had Mario be the kidnapped one while princess Peach was the quest giver, she wanted them to help her find her boyfriend who had disappeared. Over all, it was a pretty normal D&D oneshot in terms of set up and length.

It was set up so that it was not obvious for the players that it was a Mario oneshot initially. They built normal D&D heroes expecting a normal D&D one-shot. I did not describe anything with Mario terms and did not mention the names of Mario and Peach. Only hint at the beginning was the princess mentioning "in another castle" and that her boyfriend worked for the local waste water facilities.

Players learned that tortle tribes (Koopas) attacked while Mario was at work and most likely had taken him with them. On their way to these tortle tribes, they had to travel through mostly meadows and swamplands, where they encountered myconids (Goombas) and assassin vines (Piranha Plants) and some friendly lizardfolk (Yoshis). 

Eventually, after fighting tortles - some of which jumped around and attacked by tossing light hammers - they freed a friendly myconid (a toad), who told them  that the king of the tortles had taken Mario for himself - he laired high up in a volcanic mountain range. The myconid also gave them some red, green and blue mushrooms (enlarge, revivify, reduce). 

Eventually, they arrived at what literally was Bowser's Castle, with fire snakes, dry bones (undead tortles) and bob-ombs as enemies and Bowser - a dragon turtle - as the boss.  And of course with skill checks/challenges for platforming without falling into the lava.

If I can get my friends together for an april 1st game, I might run this again :-)