We have done various trips into Normandy, Belgium , Holland. Either from Hull on ferry or down to the south coast - tunnel is so easy to do , and coming back the French will usually let you on the earliest train with places left! These are our favourite trips , not having to fly , or be in bustling hotels/cities.
The WW1 sites are so moving , being at the Menin gate at 8 pm on any night is a must. Ypres is a great place to start , we took a guided tour to being with fr a good overall background, then afterwards went around [ by car and/or bike] from cemetery to cemetery. Some nice farmhouse B&B s in Belgium and Holland , though in France we now do country gites.
Also , for us , there s the farming to look at! Love seeing the Belgian blue herds , the Normandes in their native countryside ,and everywhere in these countries , a pride in the farms , no matter how small. Scenic countryside for many miles , lovely villages , chateaux/ castles , beaches , good food [ from supermarkets! so different to ours!]