Vanilla Questing is a mod which integrates quests into Minecraft while adding the least side features - UIs, commands, all that, by using advancements, goals and challenges, or just achievements for short. It attempts to make seamless quests without any mechanics other than Minecraft's integrated Advancements feature.
You can get Vanilla Questing right on this website! On this page scroll up and click where it says "Downloads", select a version and click Download.