"thinking 2 nights in Frankfurt to get acclimated and do a day trip on the Rhine.... find somewhere in the vicinity to spend 2-3 nights before going to Paris."
The ideal place for 2-3 nights is the Rhine - a day trip there from Frankfurt is completely insufficient.
So you could spend whatever time you have in mind for Frankfurt in Frankfurt, then take a train to one of the Rhine towns. Alternatively, if you don't have any particular sightseeing goals in Frankfurt proper, the Rhine towns are so close to FRA airport that you COULD easily just hop on a train at the airport train station and get there rather quickly for your first 2/3 nights - and just get acclimated there. Bingen, the gateway town to the Rhine and the place where you would want to start a river cruise, is only 48 minutes from FRA by direct train.
One Bingen hotel option, on the waterfront
Scene near Bingen
Lots of sightseeing options to the north along the Rhine - and very easy to get around by train. Run through these links to check it out:
(click on town names) http://www.loreley-info.com/eng/rhein-rhine/city-cities.php
http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php (map shows train lines and ferry crossings for getting around.)
Castles.. Rheinfels and Marksburg are the most commonly visited: http://www.loreley-info.com/eng/rhein-rhine/castles.php
