We stopped in Ypres a few times using it as a stop off between Calais and Holland. Not sure if it is at the right point for you to want a break but it’s a charming place, very traditional old Flemish (albeit much rebuilt after the war; they rebuilt what it was rather than going newfangled), easy size to wander around, lots of war history to see and do lus nice old traditional Belgian stuff.
We stayed in the Novotel - just behind the main square so you’re walking distance to do lots of stuff. Join the Novotel group rewards card online to get better rates and advance notice of sales and good offers. There was an amazing bakery opposite where we went to grab breakfast cheaply (beautiful pain au chocolat, croissants etc) rather than buy the expensive full breakfast.
Anywhere you end up - if it’s a chain, check to see what deals you get for signing up to their rewards programme. For the ‘cost’ of giving them your email and address (which you will give them anyway if you stay with them) there are often discounted room rates, things like extra perks (Marriott had free WiFi in the room)(and a promotion with MasterCard last year for free English breakfast - not sure if they are going to do it again but saves quite a bit when they want £20/adult for breakfast even if kids go free) and so on.
Holiday inn express chain also have reduced rates if you have their loyalty card and include a cooked breakfast as standard for everyone. (Or a continental equivalent thereof when abroad so the spread of cold meats and cheeses, fruit, yogurt, etc etc rather than basic bread and croissant). There was a great one we stayed in, in Arras that had massive family rooms, but others that are newer have just had standard sized rooms.
If you belong to a cashback scheme use it to book hotels and ferries etc. They often have great deals and can be used with the hotel’s own loyalty scheme - one year we went to Disneyland, got over 12% back as cashback which was waiting for us to use on our return. If you’re spending £800 on several nights accommodation and passes as we were, that meant we got almost £100 back - not to be sneezed at!
The Dutch and Belgian Centre Paris are great too and much cheaper than the Uk ones, even taking a ferry into consideration.
Leave time to stop off at the giant shopping centre with giant hypermarket near the tunnel entrance - all sorts of goodies to be had! (It’s about an hour from Ypres, as we managed to do it. Plus we got a nice selection of bits to take onto the ferry for a late lunch. Very glad we did as the ferry was delayed so it turned onto a car picnic in the waiting to load the ferry car park - we would have been really hungry if we hadn’t had much with us!
Enjoy your trip!