The World of Warcraft: Wrath of the Lich King (WotLK) Beta has many instances open and available for players to adventure in, recently Byron "Messiah" Mudry posted a guide for the Nexus, located in the Coldarra area of the Borean Tundra zone. This instance is meant for players level 71 through 73 and is a very open and free flowing design. There is no preset order in which you need to make your way through the instance, instead players get to chose their own path through the three preliminary bosses leading up to the fourth and final boss.
The instance can be completed in a little less than and hour, despite having 4 boss fights. This video shows several or the groups of creatures that you fight as you move through the instance and all of the boss fights.
You can find the preview guide to the instance here: The Nexus Preview Guide.