The most attractive building in Wiesbaden is the Kurhaus situated in the city center. Built for emperor Wilhelm around 1900 it serves as wellness and leisure time centre. It also offers a pretty garden where you can walk around or relax. Next to it are two further important buildings: the famous casino and the theatre.
The Bahá'í Temple or House of Worship - While not located in Wiesbaden, it's in a small town called Langenhain, only about a 15 minute drive away through pleasant countryside and forest, and only one of eight such Temples around the world.
He building is architecturally interesting, and has been likened to a lemon-squeezer :) If you're lucky the sun will shine through the 540 windows when you're inside - the effect can be quite breath-taking. There is also a small information centre with tea and coffee and welcoming hosts who speak English.