One week from Frankfurt to London

Having gone to Belgium now a few times, and loved it, 5-6 days is certainly not too long for Belgium, although a couple days has been my time for Brussels. Instead, consider staying in Gent (many English speakers spell it “Ghent”), which is a short train ride to the west. From Gent, you could daytrip east to Brussels, and west to Bruges (aka Brugge), and even north to Antwerp. A trip south, to Ieper (aka Ypres), could show you the area with some of the most devastating fighting in World War I, plus a very worthwhile museum involving that war, and evaluations of peace. Besides history in western Belgium, you’ll find Flemish master painter art, artful architecture, the epitome of chocolate, fries (Belgian, not French), and beer elevated to a gastronomic level.

I think most people would recommend Germany over Belgium, no slight to the Belgians

Not sure why the PP would presume that most people would select Germany over Belgium. Seems like personal preference to me. We've found Belgium to be a charming country, with plenty of things to see and do in the time period you have.

If Cologne is of interest, it's easily reached by train, and one night should suffice, on the way to Belgium.

I, too would recommend staying elsewhere than Brussels for the whole time. Ghent is a good alternative, since it is so easy to do day trips to other places, yet is also a place of interest. For us, Brussels only had a handful of interesting sites, and had all the disadvantages of a large city.

From Ghent to London, it's a quick train onto Brussels before catching the Eurostar. I'm sure you know to buy your Eurostar tickets as early as possible for the best prices.

Including a bit of Germany on the way from Cologne to Brussels, the highspeed train (ICE or Thalys) stops in Aachen. The Aachener Dom is on the UNESCO world heritage list and certainly worth a visit. The Schatzkammer was closed at the time of my visit but seems interesting too if you like early medieval artifacts. For continueing further to Brussels with the ICE or Thalys you have to book another ticket, however a local train will take longer but give you more flexibility and no booking in advance needed.

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