Wiesbaden's location in the Rhine-Main area makes it an excellent base for day trips.
Frankfurt
Frankfurt is 35 to 45 minutes away by train. The city's Museumsufer, the reconstructed Römerberg and St. Paul's Church -- where the first democratically elected German parliament met in 1848 -- are the main draws.
Mainz
Mainz, the capital of Rhineland-Palatinate, is just across the Rhine. The Roman-Germanic Central Museum, the Mainz Cathedral of St. Martin (over 1,000 years old), and the Gutenberg Museum are all worth a visit.