and stroll along the waterfront passing numerous old warehouses evoking the time when Antwerp was one of the world's largest ports - well it still is but port installations have been removed from the town center and these empty hulks remain.

So would August 12 (a Sunday) or August 13 (a Monday) be a better day to visit?

In europe not uncommon for museums to be closed on Monday

Sunday the shops would be closed so the normal vibrant town centre would be very quiet, which some would like more

If you like to visit the museums, you should come on Sunday as they're closed on Mondays. Churches are open all the time (except Carolus Borromeus which is closed on Sundays).
Personally I prefer to experience a city on weekdays during its normal daily routine.

I came back to look at this thread for information, as I will be in the area again in a few months.

I should update, as when I was in Amsterdam, I did not end up going to Antwerp on a daytrip. However, I did do two very nice daytrips to Haarlem and Leiden. I particularly enjoyed Leiden.

If someone does have this daytrip in mind, the ticket you should buy is the Smilys roundtrip ticket for 28 Euros. You have to book this at least 14 days ahead, but no more than 3 months ahead. The easiest place to buy the ticket, so far as I can see, is at the voyages-sncf.com site. I've been experimenting with the various national sites, and this one seems to work best (but you have to keep it in French). These put you on the Thalys trains with no real advantage in speed, as described above, but there is the cost advantage with a prebooked roundtrip ticket.

www.thalys.com is the Thalys web site but apparently crucial info is in Dutch?

www.thalys.com is in four languages.
Here's the Englisgh one www.thalys.com/be/en/

In any case unless you can snag the elusive SMILYS THALYS fare which exists in extremely limited numbers of many trains i believe

you're better off just buying an open ticket for the IC hourly trains that just take a little longer and not hassle with any Thalys ticketing.

you can make seat reservations if want but not really necessary, especially if starting in Amsterdam CS

Willtravel:
What did you enjoy about Leiden? My parents have spent some time there but I never learned what they liked about it. I've only driven through Leiden and am interested in spendiing time there next time.

Too bad you never made it to Antwerp as they have a good Accor downtown hotel there and I know you're a fan of Merkur (at least in Berlin).

Leiden is a neat regional town with a prestigious university that gives it a youthful zestful edge

It has romantic canals and, south part of town in the university residential district some really swell old streets.

Come on Saturdays for a swarm in the old town center - a shopping mecca with the old vestiges of the Weigh House (if i remember, etc.)

It also has some interesting museums in the univeristy area.