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 her to meet her long-disappeared father, while introducing both the players and the characters to the strange and dangerous land that is the Underdark.
The way I prepared these sessions was an interesting process. I wanted these sessions to be mostly focused on exploration and roleplay, with a single (intense) fight, as well as a puzzle. I tried to design a sandboxed environement with enough lore and backstory to make sure the players enjoy themselves and have a reason to interact with the NPCs. I wanted them to care and have the necessary space and freedom to express themselves.
Following are my session design notes, that lasted me 2 whole sessions. These were as much a way to create the world as reminders about key elements or creature capabilities that I should remember mid-fight. They ended up being quite short, because I tried really hard to paint a picture, and prepare some colorful moments, but not to anticipate my player's reactions. They mostly filled the gaps and brought life to that setting.