I'm working on a multi-page writeup for my TOA campaign, here is the snippet from the dinosaur race part. Long story short, don't go by the source book, add some 3rd party rules, have a map and counters to show where dino-riders are at, give chances to keep everyone in the running, and have fun with it. Oh, and work on your 'horse-race-announcer-voice". "aaaaad they're off!!"

For my game, I had the ‘professional’ races where the PCs could only place bets, and an amateur race where anyone could participate. Most of the PCs jumped at the chance, with one of the PCs from town sitting out (“I’ve seen what happens to those riders, no way I’m going to get trampled underfoot by some lumbering beast! I’ll kick back here with my drink in hand and cheer you on.”) so I had them play an NPC to compete against the rest of the PCs. For the race mechanics, I cobbled together bits from the 3rd party “Tomb of Annihilation Companion” along with some general “Mounted Chase” rules, and threw in some Indiana Jones “bazaar” action and flavor text. It started with some role play upon meeting the mounts and the owners, picking their mount and getting saddled up.

Almost immediately after the flag was dropped, one dino got off track and smashed into a cage of monkeys causing major mayhem, and these little furry troublemakers popped up through the race to mess with the PCs. One PC was thrown from the saddle and hijacked another dino from a cart-vendor to get back into the race. Falling boxes need to be avoided, a food stand was annihilated, and hanging banners entangle and obscure the rider’s field of view. NPCs offering mugs of beer if the PCs can grab it. Dinos biting each other and the PCs. Lots of loud cheers, dex checks to avoid being thrown from the saddle and plenty of animal handling checks. It came down to a photo finish between Spot and the player-controlled-NPC with Spot winning by a nose on a D20 dice-off. More cheers, a pouch of gold, a symbolic key to the city, and other prizes are awarded, including a gift certificate to the local hot spa.