A huge number of posters here pair Amsterdam with Belgium. I don't, because I wouldn't want to spend less than five days in Amsterdam. Because of daytrips there, you could easily allocate 7 days if you like art and museums. (There are also an incredible number of daytrips in Northern Belgium.) I did go to Cologne from Brussels, but I spent two nights, because it is a very rich destination. And probably almost as long as the trip to Amsterdam. Note also that there is more to the NL than Amsterdam. We've also slept in Den Haag and in Arnhem (with a car ... ).
I vote against London, despite the good train connection, because you've already been there. Many people here (I don't agree ... ) specify three nights for just Bruges. The cities of Bruges, Gent, Antwerp, Brussels, Leuven, Mechelen, Lier, Turnhout, and more, are very easy, unreserved train daytrips from each other. You don't need to change hotels if you are confident of your selection. (Distance to the train station matters, however!) That list is in my order of importance. Yes, I like Antwerp better than Brussels. I left out Osteende because it's not beach season in October. But there is WW II material on the coast.
Some of the many options in a place like Antwerp require research. The Middelheim Sculpture Park is a long walk from the bus stop, but very worth the effort in good weather. The giant Rivierenhof park is filled with happy families on weekends, and right by the tram. The Cogels-Osy Lei historical architecture district is often overlooked, but a treasure trove of period townhouses (exteriors only), also right on the tram or by the secondary rail stop, Berchem. Although the fabulously important Antwerp main art museum (massive Rubens collection, also James Ensor and Rik Wouters) is closed for five years of renovation, the area around it is rich with Art Deco buildings, a beautiful Synagogue (exterior), and period mansions. Very unusual, huge bronze tidal basin art piece in front of this KMSKA museum even during closure. There are still plenty of fine museums open in Antwerp, from Fashion to Photography. The two newest ones might be the MAS and the Red Star Line Museum. Antwerp has a Beguinhof, but I missed it. There is a neat pedestrian tunnel under the river, and an important shipping port (I've never seen it, it was rebuilt out of town). I found some of these in Lonely Planet.
Apparently southern Belgium has some nice hiking and green areas, but I haven't been there. I made a daytrip to the art museum in Lille, France from Antwerp, but it took longer than the trip to London. Note that the Eurostar stops in Lille anyway. Don't know if you can buy a stopover ticket.
Edit: I'd say the food is better in Belgium than in the Netherlands.
The Town Hall interiors in Antwerp and in Leuven can only be seen by, single, TI tours on a Sunday. An obscure, dusty "barn museum", the Volksmuseem Deurne, is behind a bar called (in Dutch) "The Parakeet" and only open a few hours on Sunday. It is not a must, but if you would stop in a New England barn museum, you could see this. It goes back to the time when Crown Cork and Seal was the biggest employer in that part of Belgium. Not in the center of town.
Edit 2: Aaachen is closer than Cologne, but not quite as many things to do. Duesseldorf is near Cologne, but I haven't been there yet - going in September, on the way to the documenta 14 art fair in Kassel.