We have to stop at least overnight at FRA on our way back to Japan from Paris, and thought we'd rather spend 4 nights in Mainz than the 1-night at the airport. We're very leisurely travellers and do not require lots of activities/attractions, so I'm sure that we will have plenty to do for 3-days in Mainz/Wiesbaden.
Now our plane schedules are making it easier to spend 6 nights rather than 4, but I'm worried if that might be too long. Wondering how far out of Mainz we could economically travel by train on half-day trips, leaving around noon and returning around 6 pm. Cologne seems just a bit too far for an afternoon. Any ideas for short afternoon trips by train or bus would be appreciated.
Or should we just stick with the 4 nights. This will be our first, and very likely only, visit to Germany.