Head out to Frankfurt (early morning flight on 23rd). Explore
Frankfurt?
Well, since you have a question mark after Frankfurt... while there are some who would disagree, I would suggest MAINZ instead.
Mainz is located on the Rhine River, just west of the airport. Mainz is a significantly smaller historic city with nice things to see and do that are also easy to reach on foot. When you get off the train at the main Mainz train station, it's just a few blocks to the pedestrians-only zone which leads into the old-town center. This handy map shows pedestrain zones in pink. Exit the front of the station onto the plaza in front (pink semi-circle.) Then head at roughly 2:00 toward the Isis Temple and beyond that to Market Square (Tuesday is Market Day!) and the Martinsdom (Mainz' huge Romanesque Cathedral) and into the old town zone along Augustinerstrasse (see the Augustiner church there, easy to miss) and the Kirschgarten neighborhood. There are several nice places in this area for drinks, lunch, etc.
Elsewhere... The Marc Chagall windows at St. Stephan's church are pretty spectacular and quite famous. The Stadtpark is nicely kept and great for just strolling around. The Hotel Favorite at the park's edge has a good-sized Biergarten there too.
Market Square
Kirschgarten
Augustinerstrasse
Mainz, things to see/do
Mainz Museums
The trip to Mainz and the onward journey from there to FRA will involve REGIONAL trains only and should not be purchased in advance. It's the same price no matter when you buy tickets for the regional trains, and buying a ticket early does not provide you with a seat guarantee.
TRAIN TICKETS: Your VRM Guest Ticket is valid from Boppard as far as Oberwesel. So you just need regional train tickets from Oberwesel to FRA airport. Mainz is on the way, and you can just hop off there and get back on later for the final leg to FRA whenever you like with any ticket to FRA.
The normal one-way price from Oberwesel is €53 /2 one-way DB tickets to FRA airport. You can buy this at Boppard station or using the DB app that Tuesday morning. The DB ticket is good for travel at any hour on any regional trains (RB, RE, S) heading to Mainz and FRA, and you can break up your trip with this ticket in Mainz, just as you planned.
Or you can buy 2 one-way tickets from Oberwesel to Mainz (37.40) and later two one way tickets to FRA airport (€13.80, a VRM Ticket) which saves you less than €2. VRM tickets are available at the station in Mainz or using the DB app. Note that you must buy VRM tickets JUST BEFORE you travel - they expire at a certain point. I would just get the DB tickets at €53.
When you board in Boppard, have both your Guest Ticket and your DB ticket in hand. And of course hang onto the same DB ticket for your later train to FRA airport and your hotel.
Frankfurt proper, which lies BEYOND Frankfurt Airport to the east, would of course involve extra train travel and expense because you must backtrack to your Airport hotel afterward.
Almost forgot... Your travel bags can be deposited in lockers at Mainz Hbf station while you sightsee.