I am also starting this over the weekend. I was going to make the doors zurkhwood, and the outpost observation window just be a wide opening with a good view of the slave pen. Like the press box at a football field, but with no glass.
I came up with three brief "menial task" missions to introduce the PCs to the NPCs, the drow, and the environment.
"Sous Chef" - A work team must help a quaggoth chef who is drying fish and canning entrails. Slaves must remove live quippers from a barrel, then kill and clean them with their bare hands. Some hands will be bitten, probably a guard will beat someone for not sorting the guts carefully enough. Valuable recon about where to find food if they want to steal it during the escape.
"Stacking Rocks" - Ilvara and Shoor interrupt a work team (under Jorlan's watch) that is filling barrels at the waterfall. They are super-skeevy, pawing and petting each other, and predictably nasty toward Jorlan--a good setting to hint at that whole dynamic. Ilvara orders some quaggoths to bring rocks and boulders for the slaves to stack. Progressively increasing Str or Dex check. The quaggoths enjoy watching this and gamble on it. Ilvara brutalizes anyone who knocks over the rock pile. Shoor behaves as a me-too lickspittle. I may have someone fall off the waterfall to reveal that it's not as bad a fate as it might seem. Critical success, and Ilvara gets bored. And eventually she gets bored anyway.
"Coiling Rope" - A team is taken to the floor of the cavern in the lift (good recon), where they must untangle a lot of filthy, heavy, hemp rope. Str and Dex checks to accomplish the task. Failures result in minor slashing or bludgeoning damage. Maybe a beating. When they are finished neatly coiling, a quaggoth takes the rope up in the lift, then hurls it to the floor below, where it becomes tangled again for next time. Drow don't use hemp rope.
Each of these missions involves marching a group of slaves to various locations in Velkynvelve, so they will learn how tricky the rope bridges are to negotiate, may learn about the spiders below, and so on.