The French Flemish melting pot of Lille, the fourth biggest city in France, is the shortest journey from the UK on Eurostar and well worth considering as a first hop off when starting/finishing a European adventure.
Highlights
Old Lille
Citadel
Old Stock Exchange
Station stroll ideas
If you have an hour or two to spare waiting for a train, why not go for a wander?
Tourist info have provided a set of maps and ideas on where to visit here.
I've also made a downloadable PDF and accompanying google map which will give you a feel for the place and take in the sights.
Guided tours
If you would like a guided tour, then there is a list of options here - most start from the Tourist information office.
If you are into station architecture, there is a tour (in French) which takes you from one station to the other.
There is also a network of greeters who will show you the place from their point of view for free.
Luggage
If you want to stroll without a care in the world, including your luggage, you could try using Bounce to find a place that will take your bags.
Getting there
Lille is around 1 hour 20 minutes from London, 1 hour from Paris and 45 mins from Brussels.
Useful links
Tourist info: https://en.lilletourism.com/
Where next?
How about Lyon?