If you do go Calais/Dunkirk to Ypres then as suggested Westvleteren is indeed en route. Even better and very close is St Bernardus brewery in Watou - has a lovely guesthouse (https://www.sintbernardus.be/en/guesthouse-en) you can stay in, and as of this year, a very nice roof top bar/restaurant (https://www.sintbernardus.be/en/barbernard-en)- highly recommended.
If you do go Calais/Dunkirk to Ypres then as suggested Westvleteren is indeed en route. Even better and very close is St Bernardus brewery in Watou - has a lovely guesthouse (https://www.sintbernardus.be/en/guesthouse-en) you can stay in, and as of this year, a very nice roof top bar/restaurant (https://www.sintbernardus.be/en/barbernard-en)- highly recommended.
This V2 launch site en route to it as well is incredible and well worth a visit: https://en.wikipedia.org/wiki/Blockhaus_d%27%C3%89perlecques