Lords of Waterdeep is the perfect introduction to the idea of worker placement and to board games in general. The two ways to affect other players are to block action spaces with your agents or use intrigue cards to interfere.
The game feels pre-designed to allow for low interaction, making the design of a peaceful variant simple.
1. Remove the attack category of intrigue cards and mandatory quests.
That's it. One optional change is if you are playing the Scoundrels of Skullport expansion with children. The corruption mechanic may encourage bad habits, the idea of taking extra wealth now for the risk of bad things later. You may not wish to expose children to this or it could be used a teaching tool to demonstrate the pitfalls. If it is a problem, then you not play without the Skullport module.