Some great suggestions so far! The Summer Challenge series is also perfect for a guild. The latest event brings you and your friends/guild mates to Blackrock Mountain.

We've also seen several Transmogrification contests/fashion shows on multiple realms. Maybe challenge your guild to come up with the most creative or ugliest Transmog set?

Another great example is a tour of Azeroth while the Midsummer Fire Festival is going on. During your trip, everybody can work on their “The Fires of Azeroth” and the “Desecration of the Alliance/Horde” achievements. Good luck!