Month and year? Day of Week? What is your next-day destination? What, specifically, do you mean by "shopping?" Most historic center-cities in Belgium are a 15-minute walk, or a public bus ride, from the train station. You might collect some taxi phone numbers, just in case. All domestic tickets to Brussels include all three downtown stations (not the airport.) So find out where your lodging is. Of course there are taxi stands at rail stations. Remember REQUIRED paperwork/security time if you are boarding Eurostar in London.
You are wise to prepare, but it's not as tricky as you suspect. Because these Belgian domestic trains operate more like a commuter line, unreserved and around five trains an hour at midday, you have great flexibility. If you don't want to risk a line for the ticket, you could check the add-on cost of "Any Belgian Station" for your Eurostar ticket. It can be tedious to wrangle a big carry-on upstairs to a double-decker commuter train car, with zero "long-distance" luggage racks.
I personally like Antwerp, but the overwhelming vote on this board (and people you'll see at cocktail parties back home) is for Bruges. I mention Antwerp, because (like most places in Belgium), it has a local, artisanal chocolate shop, Burie. No idea about classes, which are Team-Building and tourist fare. But Antwerp has two branches of Desire de Lille. a gourmet waffle restaurant. They also have Smoutebollen, kind of like fried dough but softer and made with a yogurt-like milk product. Toppings include chocolate or berries.
Bruges is bigger and older, but if your companion is having a bad day, Ghent is more compact. Walking is necessary in both cities. Google Maps is your friend. The Mystic Lamb paintings in Ghent (Monument Men ... ) requires advance booking. Bruges is VERY crowded in the summer.
