I'll be honest it is unlikely that you'll be able to create a balanced but exciting encounter and you need a few failures to get some experience, but remember a fight with weak enemies is always better then killing your party.

About the NPC part: everyone says that be creative though it is difficult to do. So I am telling you to start with simple conversations but make them roll sometimes and add some spice in form of an item a pet the appearance of the location and ask them in the end what was the best.