PalenQ

How far in advance should we book the rail pass (baln.eurorail)?>

that's www.bahn.de/en - you should compare prices and specials for agents in your own country vs bahn.de prices - take whatever is best - they are not always the same and can go either way. Also check for specials on either end - like a free day of travel at times - usually off season however.

Mimar

Important! Do you have your plane tickets yet? If not, buy open jaws tickets into one city like Amsterdam, out of another, like maybe Berlin or Frankfurt. Open jaws tickets (called multi-city on airline web sites) should cost about the same as round-trip and save you time and money spent backtracking to your arrival city.

dreamon

What are your plans for Frankfurt? It's a less common choice. We stayed in Heidelberg and enjoyed it so you could visit from Frankfurt if you wished.

With three cities, I'd consider choosing a smaller town for your fourth destination - or just stick to three and add some day trips. Maybe Utrecht or Maastricht or somewhere more rural? No doubt there are heaps of lovely small towns between Berlin and Frankfurt but it's not an area I'm familiar with so will leave to others to suggest.

If you have specific questions which seat61 doesn't answer, come back and ask.

PalenQ

Heidelberg is great - one of the few larger German cities not blitzed to bits during WW2 - some say because the allies planned to use it as their post-war HQs which the Americans did - there are still lots of American GIs in the area. But a neat castle hovering over the town - neat Altstadt with famous student pubs (large university town) - a neat place for a leisurely day.

Sassafrass

Why has Frankfurt suddenly appeared on the itinerary?

Honestly, I lived there for three years, and if I happen to have a few hours layover there, I enjoy a quick visit, but I would never deliberately put it on the itinerary when there are so many incredible places to visit instead.

Frankly, IMHO, it would be way near the bottom of any list of interesting places to see in Europe.

StCirq

Frankfurt? WHY? I've spent a major percentage of my life in Europe and Frankfurt has to be one of the least desirable places I've ever set foot in. What guidebooks and internet sites have you been reading that lead you to think it should be a focus of a very short trip to Europe?

Sure, it's Europe and it's got a few things to see and do, but honestly, with ALL of Europe at your fingertips, why Frankfurt? It's like making a once-in-a-lifetime trip to the USA and deciding that Rapid City is on the bucket list.