Arriving in the Brussels Airport, you just follow arrivals, and at the customs, just focus on the inner lines, as the others are for european passports. Once in the arrival hall, you go to you 2 o'clock, walk a 50 yard, where you have escalators bringing you to level '0'.
Exiting the building you will see bus stops. You want to take the Airport Express (dark blue - white) which is a little to the left. It leaves at the hour (7am, 8am, 9am,...) But is sometimes late, so ask people there since when they are waiting. The ride is euro 10, but you can also pay in USD, but not with plastic.
Bus ride takes approx 50 mins, and you are getting out at the second (and last). stop.
(In case you just missed the bus, return to the airport, and go to level '-1', and take a train to Antwerpen. Buy just a one way ticket. You will have to change train at Brussel-Noord (north) station, which isn't a big deal, but beware of pickpockets), and get out at Antwerpen Centraal (Antwerp Central [station], and ask for the Keyserlei (streetname))
Normally this bus should stop right in front of the central station (on keyserlei), but as they are working there, they might stop somewhere else in immediate vicinity, just ask anyone for the Centraal Station.
The station is a beautiful building, most likely the nicest station in the world. There is a Starbucks, but I would only buy only a regular drink such as Latte, hot chocolate, etc, as the rest, they get locally (not OU).
After the station, leave the place going left, (along the rails) (pelikaanstraat), and turn very strong right into the Vesting straat. You will see here lots of jewelry stores. Take the first left onto the rijftstraat (where the diamond area starts)and follow the street. It will turn left into the hoveniersstraat, , and then finally turn right onto the schupstraat. On your right side you will see a cafe called Eden, that serves light diary food, such salads, soups, toasts etc. Take the first right onto the lange herentalse straat, and you will see immediately on your left another restaurant called Time Out, where they serve Pizza, filled baguetts, etc. (You might consider take way at this place. Exiting the restaurant on left (continuing your way) the street will change name and be called appelmanstraat.
The first street to the left will be the keyserlei, stroll down the street, cross a big crossroad, but continue your way.
(The street is now called Meir) pay attention because some of the house are master pieces. After a short while the whole street turns left. At that point on your left (meir 78) you have the entrance to the Stadfeestzaal, which is a shopping center, with its wow-architecture more interesting than its sales.
Exiting the shopping center, walk left until the road splits to two, and you choose left.
On your right you will see a big open place called Groenplaats , which you cross to the furthest corner (right front).
There a little street will lead you another place called Grote Markt.
You can stroll around the little side streets, with lots of houses that are up to 400 years old.
To head back: if unsure just ask for the Meir, and walk direction of the Centraal Station.
Taking the bus back to the airport: the bus also here leaves at the hour (3pm, 4pm, 5pm, etc).
As you are checked in already, if you have your boarding passes, an hour is theoretically enough, however, I would suggest to make it 2, or at least 1.5 hour, as you never know how many people are waiting at the security checks. (If you think you are running late, let me know, I have a way to get you faster through the checks, only if in trouble for the latter)
If you see that you have spare time, and want to see more stuff, or have anything you need really, (no need to be an emergency), please contact me.
Enjoy your stay, and happy travels !