I know it's probably too late now, as the Op said they'd be travelling today.
Anyway, here are my deux centimes.

I'd also do Boulogne, but if you're more adventurous go to Ghent. Park under the Marriott, emerge from the hotel to the most wonderful view.

I think the Auchan on the St Omer road at Boulogne is good, but pricey. The F LeClerc in Boulogne is better. However, since the Op is going ot Cite Europe, they might as well do the Carrefour.

Remember that there's quite a good Saturday morning market in Boulogne, better (in my opinion) to the St Omer version.

Need to check your times as you lose an hour going to France. Don't expect to arrive 35 minutes after the scheduled departure time. It'll be 1:35.

Have fun.