You can buy this adventure right here.
A new batch of Adventurers League modules came out, all of which are linked to Tomb of Annihilation. A Day at the Races is the third one, and it primarily deals with a deluxe version of a dinosaur race.
I've read this whole thing and I can say with confidence, if you plan on doing a dinosaur race in your Tomb of Annihilation campaign, buy this and use the event in here. It is tons of fun and full of really great ideas. It sort of kills me inside that my group has already left Port Nyanzaru, as there's no way for me to run this.
Three Parts: This adventure is broken up into three sections. Part 2, the race, is by far the largest section and makes up the vast majority of the adventure.
This dinosaur race event is called Na 'Nbuso's Haka. It is meant to honors kings of Chult's ancient past.
Story: There's been some suspicious stuff going on in recent dinosaur races. Fluke wins, freak deaths, things that have many people concerned that there's cheating afoot. The heroes are going to enter a race and try to figure out what's going on.
Part One: The group registers for the event. They must pick which team name\uniforms they want (there are ten choices). The team choices are based on different creatures in Chult: grung, froghemoth, t. rex (!) and more.
The heroes can gamble on the two races that occur before theirs.
Part Two: Ok. Ready? This race occurs in four phases: Land, air, sea, and grand prix royale.
First Leg: The group rides on chariots pulled by dinosaurs. We're already done here, right? This is the best thing ever.
Second Leg: The heroes ride pteranodons (!)through the air, attempting to swoop through wide rings of iron. Each rider needs to snatch a ribbon from a ring of iron. Once you get a ribbon, you can harry riders on another team until your entire team has ribbons (at which time the group can advance to the next phase).
Third Leg: The teams must ride on plesiosaurs in the ocean toward a statue in the water. Once at the statue, the heroes must dive in, swim down, grab a brick, get back on the plesiosaur and take it to the warehouse district. Then they need to solve a puzzle.
The mind reels from all of the cool things you can do with this. Imagine you're diving down to get the brick at the same time that a rival is. Underwater brawl? Shark attack? Playful dolphins? Aremag appearance?!
Final Leg - Grand Prix Royale: The entire group rides on one triceratops and races to the finish. The characters each have a shield and a whip, as do the opposing teams. You all whip the crap out of each other.
The entire team on one mount. Fantastic! So much hilarity can come from this, so many cool maneuvers.
Part 3: This is a quick conclusion to the scenario. Some creatures run amok right after the race concludes. The heroes take them down and then they discover a container with a really, really cool substance in it that I don't want to spoil. This discovery leads into the next adventure, A Walk in the Park.
I love this adventure. Love it! I'm not sure what else there is to say. If you're running a dinosaur race, you have to use this. It's almost cruel not to. If players knew what they were missing, they might try to punch you right in the face and, honestly, I can't blame them. How dare you! Just who do you think you are?
As far a nitpicks or concerns, I do think that some dungeon masters might want more concrete rules in the race section, as a lot of details and rulings are left up to the DM. I could also see new DMs having trouble running the pteranodons-flying-through-the-rings section, as it's quite vague.
I would have also liked a description of some rival NPCs and teams.
But those are minor nitpicks. This is easily one of the best releases linked to Tomb of Annihilation so far. You should definitely check it out and see if it's something you want to use.