My wife and I,together with our adult daughter.will be staying in St Goar on 28 Sept - 01 Oct meaning that we will mainly have the Saturday and Sunday to do whatever.
We want to visit and experience the wine festival and fireworks in Boppard on the Saturday - planning to travel there by boat from St Goar and back by train.Should we reserve the whole day for this or would it be sufficient if we go there in the afternoon?
We were thinking of traveling by train from town to town on the Sunday in order to explore Mainz and then to return to St Goar either by boat or train depending on schedules and time needed to explore e.g.Bacharach,Rudesheim and Mainz.
Is the above a viable and practical schedule or is there anything that I should rethink?
"...planning to travel there (Boppard) by boat from St Goar and back by train.Should we reserve the whole day for this or would it be sufficient if we go there in the afternoon?"
If you're going to cruise, then cruise the best part also, not just the skimpy St. Goar - Boppard segment... Bingen-St. Goar is imperative:
Let's say you travel first to Bingen by train (as you should) and cruise north from there at 10:30; you could just get off in St. Goar, have lunch or whatever, and continue by train to Boppard (around 10 minutes away.) Music and wine stands on Saturday get started around noon and continue until the fireworks at 8:45 so you can get to Boppard whenever you please, really.
If you absolutely want to cruise St. Goar - Boppard as well you could either stay on that 10:30 boat, or hop off in St. Goar for a while and catch a later boat to Boppard (tell crew when you get off in St. Goar about your plans so they will mark your ticket.) Note however that the cruise fare to Boppard is €8 more each than the fare to St. Goar.
As for the train fare, a Rheinland-Pfalz ticket costs €34 for 3 (buy at station from ticket machine) and provides unlimited use of the trains all day long - no additional costs for any additional train trips.
Another option would be to visit Bacharach for a couple of hours on your train trip south to Bingen that morning... then continue later by train to Bingen for your cruise back north to St. Goar (or Boppard) on the 14:30 boat.
Rüdesheim: You might want to see it and Marksburg castle in Braubach on a separate day using a separate Rheinland-Pfalz ticket; note that the St. Goar ferry crossing gives access to the opposite river bank, where these towns are located, with the R-P ticket.
Mainz is very nice but kind of a long day trip from St. Goar. You might instead visit Mainz on your way to or from the St. Goar area. Mainz station has lockers - you could spend the whole day or a few hours there, then move on to somewhere else, or stay there a night. Mainz is great as a pre-flight or post-flight stop as it's so close to FRA airport.