If you drive to Koblenz, are there l/2 day (two three hour) cruises on the Rhine that you can just show up and get a ticket? I've heared that Koblenz is one of the best places to start a short boat ride down the Rhine River.

Yes, as long as you join the queues early in peak season. This is the most popular (and slightly more expensive) company: http://www.k-d.com

Koblenz is a good starting point, as is Boppard. You can then go to wonderful Bacharach for instance.

yes you can . but many years ago there was a road that ran parrallel to the river along the side of and between the ridges that are almost parrallel to the river. there use to be one on the boppard side of the river. the road is higher in many places that the towns and castles on the river and there were places to stop and take pictures and eat some food you brought with you.. also the views of the towns and castles we liked better than from a boat on the river because you were looking down at them. we took a pair of field glass and a telescope and it was like looking at them from high up in a helicopter at many places we stopped.. .
i hope for you the road is still there.

Yes but just get up the heights at Rüdesheim or so for that effect. The boat is still fascinating as it is such a current in the river and a great difference between upstream and downstream travel :-) Also fascinating is the navigation in the many sharp bends in the river.
But yes, it IS nice to see it from above too.