How to spend one evening near Weisbaden

I don't know what you call travel worthy, but a hundred euros sounds like a lot for an evening. I would pass on it and find a bar/cafe not on the cruise curcuit. You'll have more fun, probably, than taking part in an orchestrated cruise evening. I haven't been in Rudesheim but I suspect you can walk to a more interesting place.

Weisbaden is a very "classy" city, the provincial capital. So there is good nightlife there, including a casino. Across the river is Mainz, an even bigger city with lots of cozy little wine bars, some great old churches and a walkable old town. If the ship docks in Rudesheim you are about 30 minutes away from Weisbaden.
Rudesheim is a bit of a tourist trap and on lots of tour curcuits.

Viking wants to charge you €100 for something you can do yourself for a fraction of that cost? Rudesheim is very touristy, but I think it's worth visiting for at least two reasons. One, it's a very pretty town in a scenic location. Two, where else in Germany can you find so many restaurants with live Schlager performances? ("Oom-pah" bands, by contrast, are specific to Bavaria, not this region of Germany). But if you want something a little more sophisticated, I would second the recommendation for Wiesbaden. It's a very elegant and lively city.

That is so over-priced it is sad. Was that a per person price? If you did want to see Rüdesheim, I would go without the tour. Take the chair lift up to the Niederwald Monument for the best view. Cost is 7 € round-trip. There are any number of wine cellars in this town that will be happy to serve you a local schnapps, which I am guessing is Asbach Uralt, and there are lots of live bands here too. Even the train that goes up to the Niederwald can be ridden on your own. Wiesbaden is absolutely gorgeous. Go down to the Marktkirche and the Marktplatz. Wonderful pedestrian zone too.

I would also suggest the chair lift up to the Niederwald monument. One of the most beautiful rides in Germany. 100 Euros for a kitschy tourist evening is ridiculous. If you want a classy evening on the Rhine, try the restaurant Schloss Johannisberg: http://www.schloss-johannisberg.de/de/rtd.speisekarte.php There are a ton of nice restaurants in many of the villages near Rudesheim. For a more real life German expereince, go into Wiesbaden or Mainz.

Thanks to all for the good advice. It's true about the price. In Viking's defense, the price is 50 Euros per person, they bear the cost of transporting a fair number of people on each event, and they plan it all - which for some is the reason to pay more. Since that's not me, my thought was to wander through the most interesting parts of the city (whichever city) as museums and most churches will be closed, seek a really fine dinner, a spot away from the heaviest tourist crush - and try some great local wines. And I think you're all suggesting the music may be new to me, which is great. What I'm sorriest to miss is in Mainz - the Chagall windows, the Dom there, the museum of ancient shipbuilding...

"seek a really fine dinner, a spot away from the heaviest tourist crush" If you're still talking about Rüdesheim, don't worry about your fellow tourists. I've taken visitors to the town several times, and I've never had a bad meal there. And the places that don't cater to tourists (if there are any in Rüdesheim, besides perhaps a döner stand) probably don't have any Schlager bands. But if you're talking about Wiesbaden... there's not much of a tourist crush there to begin with. It's one of the wealthiest cities in Germany, so I expect a subpar restaurant could not last very long in Wiesbaden. " - and try some great local wines." Hate to burst your bubble again, but that area of Rhienhessen is known for making cheap table wine that you see in the grocery stores for about €3-5/bottle. Now personally, I like the cheap grog they make around here, but I've been told some oenophiles consider the wine inferior.

LOL! The bubble is burst - and all over the web are notes about the local 'wine country'! We don't claim to be wine experts, we were just hopeful.
Taking all you've mentioned into consideration...

I really enjoy the town of Oestrich Winkel which is on the way from Wiesbaden to Rudesheim. It's a cute, small town on the Rhine that has several old half-timbered buildings. I had a wonderful German meal at the restaurant, Gruener Baum, which is in the middle of the town. Have a great trip!