I am going to be travelling in mid-July with 2 children (10 & 14). I would like to take a boat (not a cruise) from Koblenz along the Rhine with the capability to off at the various towns. Do you know if this service exists? Also, I would like to spend a night or two at one of the towns. Can you recommend the best town to relax where there are rental bikes, a swimming pool and other kid friendly activities?
If by "not a cruise" you mean not on a boat with overnight accommodations, the K-D boat trips would satisfy that requirement. However, if you look at the schedule, there are only five trips up the Rhein from Koblenz each day, about 2 hours apart, so you can't do much hop-on/hop-off-ing.
I can't tell you much about bike rentals and swimming pools, but the Rheinfels castle ruins in St. Goar are a great place for kids, particularly boys. Also the intact Marksburg castle at Braubach is a good look at a real medieval castle. The K-D boats stop at both Braubach and St. Goar.
Check www.k-d.com or www.bingen-ruedesheimer.com for information on boating the Rhein. St. Goar is a great home base. Check their site at www.st-goar.de for tourist information…click on leisure activities for information on swimming pools, etc. I can recommend Hotel Hauser for accommodations right on the river… http://www.hotelhauser.de/