Brussels--which station?
 

Taking the Eurostar for the day from London..

It terminates at Midi, but I believe i read here that it stops at all Brussels stations. Could somebody please tell me which would be the best to be closest to the center of town--say, to Grand Place?

eurostar only serves Midi (aka Zuid) from there you can hop frequent trains to Gare Central - a few blocks from the Grand' Place - with your Eurostar ticket you get a free ABS ticket (All Belgian Stations) meaning you could save a euro or two by using it to get to Centrale station. Ask about the ABS when buying your Eurostar ticket.

Hop on a train to Gare Centrale, exit the station from underground and walk down hill toward the spire of the Hotel du Ville while following signs toward the Grand Place - Grote Markt. It's a nice 10 minute walk.

i already bought my Eurostar ticket, so I can't inquire about ABS. :-( Do you buy tickets at a window, or is their lovely little machines like most other cities have?

I believe that your Eurostar ticket is good for the short hop from Midi/Zuid to Central, but the people who know will correct me if I'm mistaken.

In short, no need for you to do anything.

No need to do anything beyond hold onto your Eurostar ticket to show if ask. I've usually done this switch with ICE or Thalys tickets, but I'm sure it's the same.

Trains leave Midi for Central every 10 or 15 minutes (tho the destination is always somewhere beyond so look at the 2nd stop on the itinerary)and takes less than 15 minutes. I just stand inside the door as not worth taking a seat.