I would stay in charming old-world Boppard - several miles south of Koblenz - rather than in larger, more modern Koblenz. Of the smaller Rhine towns on the west Rhine bank, Boppard has the best all around choice of accommodations and dining options.
Boppard has a chairlift to a scenic overlook. From there you can do some easy walking on the trails and take in some great views. The Vierseenblick (4-lakes-view) makes for a unique picture opportunity. It's a spot where the Rhine appears to be 4 lakes rather than a river. If your unfit friend tires easily, there's an outdoor restaurant with stunning views called Gedeonseck, just a few minutes walk along the trail that leads from the top of the chairlift.
To the south, you can visit nearby Oberwesel nearby, which has a fairly easy in-town walk around the old town wall:
You might visit St. Goar as well, even closer to Boppard than Oberwesel. There are some trails atop the cliffs high above St. Goar with stunning views as well. You can drive up to St. Goar-Biebernheim, park, find the Rheinburgenweg (Rhine Castle trail) and follow it south to the "Loreleyblick Maria Ruh" lookout, where you have a direct view of the Loreley cliffs on the opposite river bank. (Walking up there from St. Goar is quite strenuous.) On the way, you may want to park at Rheinfels Castle and take a tour there. The views are great from there as well. And the hotel next to the castle has a nice terrace restaurant called the "Landgraf."