Schönes-Wochenende-Ticket for FRA-Boppard stopping in Mainz-confirming I can buy @ airport?

You just buy them out of a vending machine. Not quite as easy taking money from an ATM, but close.
I'd have to look at this for a few minutes, as there may be even better deals.

Are all four people adults? Another option would be a local, RMV, ticket from the airport to Mainz and then a Rheinland-Pfalz-Ticket (39€ for 4 people). The Rheinland-Pfalz-Ticket would cover unlimited travel in the state (Land) of Rheinland-Pfalz, so would cover Mainz to Boppard and back.

However, the Schönes-Wochenende-Ticket is only 52€ for 4 people, 13€ more than the RL-P ticket, and covers the entire trip. The price of the RMV ticket to Mainz depend on the number of adults and children, but will almost certainly be more than 13€. So the S-W-T will almost certainly be your best bet. And it's more convenient to just buy a single pass.

The S-W-T is always the same price; you don't need advance purchase. It is 2€ more at a counter if you can't figure out the automat (not hard; it can be switched to display in English).

thanks. 2A + 2 teens (15), which I think are "adults." We might divide up the ticket, as you suggested, in case we get to the Mainz hotel and then are too jet lagged to venture farther than Mainz on arrival day. How much is the RMV for 4? I only see 36 euros for some trains online, but i am guessing there is a cheaper local fare?

RMV one-way tickets from FRA to Mainz:

Adults: €4.85 each
Children 6-14: €2.80
Children under 6 = free

Four adults (over 14) is 4,85 x 4 = 19,40€, but an RMV Gruppentageskarte, for up to 5 people of any age, available from an RMV ticket automat or from a counter, is 15,40€, so it would only cost you 2,40€ to wait on the ticket all the way to Boppard.

The 36€ tickets you saw were undoubtedly for an express train (ICE, IC, or EC). RMV runs the regional trains and the S-Bahn in the Frankfurt metro district. Since they sell those tickets, the Bahn doesn't sell the tickets or show prices. Note, the RMV ticket is only valid on regional trains or the S-Bahn. There are two stations at the airport. The Regionalbhf is in close, under the building across the access road from Terminal 1. The second station, the Fernbahnhof, is out beyond that building, on the other side of the autobahn. Regional trains us the Regionalbhf.

Incidentally, there are several stations in Mainz. Where is your hotel? There are two S-Bahn going through the Regionalbhf at the airport, S8 and S9. They alternate, 15 min apart. The S8 goes to Mainz Hbf and stops at most stations in Mainz. The S9 goes to Wiesbaden and only stops at two station, Bischofsheim and Kastel, in Mainz.

we are at the hilton city (great rate+free breakfast)--the one by the main station. So, it sounds like I can buy the 15 euro ticket to cover all of us using the S and it is the most convenient for our flight anyway and that would go to the main station?

Looking at the map on their website, the closest station in Mainz to the Hilton appears to be the main station (Hauptbahnhof). Google Maps says it's about a 9 min, ½ mile (700m) walk from the station. You can probably find a taxi in front of the station.

For future reference, Hotel Königshof is right across the Bahnhofsplatz, less then 100 yds from the station. I stayed there in 2008 and thought it was nice and convenient. They have a great breakfast buffet. When I was there they had a section with American food, including bacon.

I've also stayed down the street at Schottenhof, but prefer Königshof. Next door to Königshof, slightly closer to the Bahnhof, is Hotel Hammer. I haven't stayed there.

thanks. We can make the 700 M walk with rolling suitcases and 2 teens @ 11AM. We are staying on (low) HH points for a family room. We were going to stay In Boppard, but essentially free @ Hilton won. We will just use Mainz as a base for day trips for 2 days.