For Belgium I would suggest a start at Gent. Much nicer place than Brugges and not so many Brits about. There are a number of sites around the city - we stay at Blaarmeersen which is next to the national watersports centre. Big site, shop, restaurant, bistro, good facilities, and a bus stop into the city almost outside the gate. It is also only about 40 miles of free motorway to Lille for a dip into French.
From Gent if you are interested in the battlefields I would go for the municipal in Ypres. Good site, 10 mins walk to the Menin Gate and the city centre.
From Ypres my next stop would be in the Ardennes - Dinant is a useful place and has a couple of sites. Alternatively somewhere in eastern Belgium around Liege whence you can troll off into the northern Ardennes and (as we did) trip over the F1GP circuit at Spa Francorchamps which is worth a look and has free access (or it was) - that hill at Eau-Rouge just has to be seen!
Finally Luxembourg: the city is worth a few days, has interesting shopping and a good museum. We stay at Kockelscheuer a bit to the SW. Good site, shop, bistro, sports centre with pools next door, and spotless heated facilities. Bus stop into city about 200m walk. It is also a viable day trip to go into 'Little Switzerland' in the north of the Duchy, places like Vianden.
As someone else has mentioned the eastern border area such as Echternach is very pleasant. For a good site try the municipal at Born about half way between Echternach and Wasserbillig - the old border crossing before the autobahn was built. Good modern facilities, restaurant/bar on site, and quiet. It is close enough to the German border to not only visit Trier but also Piesport and Bernkastel-Kues on the Mosel.
On the way back go to the old capital of Holland, Mechelen, and Antwerp, then Knokke (as in the Golden Rose of..) and down the coast back to the ferry. Many people never realise what a beautiful sandy coast Belgium has - to the extent that there is a tram line all the way from Knokke to Ostend. Knokke is rather - shall we say - up market.