I've recently designed a 2 session long (6h) detour into the Underdark, that would feed into one of my player's character's backstory. The goal was to allow him to meet his long-disappeared father, while introducing both the players and the characters to the strange and dangerous land that is the Underdark.
When I was preparing for Port Nyanzaru, in Tomb of Annihilation, I started reading what other Dungeons Masters had to say about the city. A lot of them would mention that the dinosaur race was a must-do, and that if done properly, it could really be a high point in the start of the adventure. The problem was, I felt that the official rules regarding this race were, well, underwhelming, to say the least. Each player rolls a dice, gets some points or not, repeatedly until the end of the race. If that race was going to be something to remember, I felt that I needed to spice it up a bit.