When at the Hotel Schoenburg what is the best way to see the
area....we have two nights
Auf Schoenburg's clifftop location makes coming and going difficult without a car or a taxi driver. Once you're down in Oberwesel proper, however, it's very simple to take the train to Bacharach (5 minutes south) or north to St. Goar (5 minutes, Rheinfels Castle tour) or Boppard (10 minutes, cool chairlift ride and more, see links below.) Trains run hourly or more frequently in each direction throughout the day.
https://www.boppard-tourismus.de/media/was-ist-wo-broschu__re_en_190218.pdf
You do not say WHEN you will be on the Rhine; most tourist activities and cruises start up around Easter.
Slightly further from Oberwesel: Marksburg Castle makes for the best medieval castle tour in the area. Train to Koblenz, change there for the 10-minute ride to Braubach.
https://www.marksburg.de/en/
Map of area w/ town links, ferry crossings, cruise docks, railways:
https://www.marksburg.de/en/
Cruise: I would caution against cruising south from Koblenz to Oberwesel if you want to use the small amount of time you have for seeing the area efficiently. Southbound cruising bucks the very strong river current - and you will need nearly 5 hours to reach Oberwesel. Also, the scenery to the north of St. Goar isn't nearly as charming as it is between St. Goar and Bingen. What to do? After breakfast at Schoenburg, take the train from Oberwesel south to Bingen, then board a northbound boat to St. Goar or St. Goarshausen (1.5-hr. cruise through the best part.) Then...
1) Get off in St. Goar to visit Rheinfels Castle and catch a train "home" to Oberwesel. OR...
2) Get off in St. Goarshausen, catch a train to Braubuach for a tour of Marksburg Castle - then train home via Koblenz.