Robespierre

Following along with the split trip idea - leave Paris earlier (we arrived in Köln before 11AM), stash your gear at the Hauptbahnhof, and spend the entire day around the Nordrhein area. You could see the Dom and sights Hans listed without leaving town, or take a quick trip up to Neanderthal to see the museum and excavation site, or down to Königswinter to ride the rack railway up to the Dragon's Rock.

Then go on to Berlin on the 19:49, arriving just after midnight.

WillTravel

Having taken this exact Paris-Berlin EasyJet flight (with one teen only, though) followed by public transport, I think I can safely say it is not grueling. I suppose if it was the rare circumstance where something went awry, that could be stressful, but most likely it will be over quite quickly from start to finish.

Clinpsych

Thanks everyone. Your advice is invaluable. Am feeling much better about Paris to Berlin now and you have saved me from going bald!