For art museums, Cologne is a great choice. Frankfurt as well.
If you want to mix that with something "quaint" not too far off your route you could check out Münster. Also many water castles in that region.
Hamburg should also fit your tastes, but may be a bit too far away.
Hanover is not terribly exciting.

Thanks Cowboy. I'll check out Munster. I was also thinking of doing a daytrip (from Frankfurt) to Darmstadt to look at the Art Nouveau architecture there.

Hi outwest, just a heads up about the spelling - there is both a 'Münster' (pretty northern city) and a 'Munster' (where the British Army used to be stationed). If you are searching for 'Münster' in search engines like www.bahn.de and don't know how to do the umlaut (the two dots), do a 'ue' instead.
So
ä = ae
ö = oe
ü = ue
ß = ss

It's easy when you know the trick!

Lavandula

I didn't find Hanover that great, outside of the Herrenhäuser Gärten it was just okay. We really went to Herrenhauser because of the Nikki de St Phalle cave. I like Hamburg and Bremen there's a lot in the area that kept us entertained for 2 weeks. Hamburg had wonderful art museums. Small but very good.

A lot of people just seem to like to see the "big sites" and after that they lose patience and there's nothing wrong with that.

I loved Rotterdam and Brussels. Koln has a great museum, plus the cathedral. We spent almost a whole day in that museum.

Munster is high on my list and if you go there, stop off at the Kroller Mueller museum. It's and art experience that is well worth your while. It has a nice collection, a very large Van Gogh collection and a sculpture garden that is perfect for strolling.

Plus, you'll be going through the Hoge Veluwe in April/May and the rhododendrons will be in full swing. Do a search on the park itself, you may be pleasently surprised.

Thanks Lavandula—I'm still a little confused. Would the train station I want be Muenster (w) Zentrum? or Muenster (west) hbf?

Lsky—we actually went to the Kroller Muller museum a long time ago (maybe 20 years?) It still stands out as one of the great museum experiences!

I confess my last visit to Münster was 1990 (and I do remember it, but it was a long time ago) - but looking at Google Maps I could only see Münster West Hbf, which is pretty close to the main square, at least within a short taxi ride. In German towns the centre is always where the town hall is, on the main square.

Lavandula

Thanks!

If you are into art, Dusseldorf is a must - great museums (K20, K21, Kunstpalast, NRW Forum, Julia Stoschek collection, the latter only if you like really modern stuff) and certainly Germany's most significant academy of arts.

Also, the city has some nice modern archicture, e.g. by Gehry (Media harbour district).

Just a clarification:
The main station of Münster (the one you wish to use as destination) is Münster (W) Hbf.
The "W" is an abbreviation for Westphalia or Westfalen in German and does not stand for west as in Western part of Münster.

You should include Bruges in your Belgian visit.Antwerp-Ghent and Bruges lie on the same train line.Antwerp-Ghent is about 60 km and Ghent-Bruges 55 km by road.To get an idea you should count on about 3 days in each location.Paul

Cologne is a great destination for anyone interested in art and architecture. In addition to the cathedral, it boasts twelve Romanesque churches. I wouldn't take it off your list.

Frankfurt has excellent museums. Mainz, on the S-bahn system and very easy to reach, has a major Romanesque cathedral, the Gutenberg museum, St. Steven's church with windows by Chagall and a pleasant Altstadt.

My husband and I really enjoyed a long day trip to Antwerp and wished we had more time there. I highly recommend two wonderful museums, the Museum Plantin-Moretus and the Museum Mayer van den Bergh. The first is a fascinating 33-room museum showing the home and printing plant of a family who ran a major publishing house for several centuries, beginning in 1576. The second contains a large and impressive collection of paintings, sculpture, stained glass, tapestries and manuscripts amassed by an art connoisseur in the 19th century. The highlights are two works by Pieter Bruegel the Elder, a set of miniatures illustrating Twelve Proverbs and a major work, Dulle Griet ("Mad Meg")

since you are interested in Art Nouveau sites, be sure to visit the Grand Cafe Horta which is built with parts of Horta's Maison du Peuple.

Thanks everyone. We visited Brugges a year ago and liked it but probably wouldn't go back. We're going to Antwerp because we spent one night there and wanted to see more.

I'm thinking 3 nights Cologne, 4 nights Frankfurt—possibly with a day trip. I just can't decide whether we should allow 2 nights in Dusseldorf at the start of our trip. I'm leaning towards yes.

if you are going as far south as Cologne [Köln in german] you might want to travel to Antwerp via Aachen and Maastricht. Both beautiful and historic cities well worth a night or two.