All three of these nordic passerines are, while regular, scarce and good finds in Belgium. Lapland bunting is more a migrant in october, less a winter visitor. Snow bunting is easier to find then Twite for me, not because of numbers, but because they are more predictable (sea side of dikes, thin vegetation or debris between stone or concrete pavements), and they are a lot easier to recognize then twite
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Since your starting point is near Brussels, I would still consider the Dutch delta, the south east part is actually closer then the Belgian coast.
There is also a wealth of info for birding in the Netherlands, Belgium still has some catching up to do in that regard.
For exemple distribution maps of snow bunting and twite:
http://waarneming.nl/soort/maps/329
http://waarneming.nl/soort/maps/249
Back to Belgium, other interesting places near a harbour, Antwerp in this case, is the following string of recent "harbour compensation" nature reserves:
http://www.natuurpuntwal.be/natuurgebieden/index.htm
These are my personal favorites in this list:
http://waarnemingen.be/gebied/info/28704
http://waarnemingen.be/gebied/info/29908 (follow Haandorpweg left to the dock)
http://waarnemingen.be/gebied/info/29902 (perhaps better in spring)
Not a reserve, but while you are in the neighborhood:
http://waarnemingen.be/gebied/info/29880 (follow the dirt track to the dock)
Waders are still around in good numbers, but less species then during migration. Ducks and geese are normally the highlight in winter, and are easier to photograph. For photography, the northermost hide of Uitkerkse polders can be good (best light in morning, backlight in evening)
This reserve has a good hide:
http://waarnemingen.be/gebied/info/29934 (southwest corner, follow the dirt track parallel to the main road, best light in afternoon)
The south shore of the Westerschelde is a long time favorite of mine, it was only a half hour away where I used to live. The area I visit most is between Hoofdplaat and the harbour of Terneuzen. Had snow bunting there last year. There is a road along the dike, if you climb on the dike every few km you get a good overview, it's easy to find the good spots that way.
The Braakman nature reserve is nearby, and worth a visit. This part of the reserve has a hiding wall on the shore, with photo opportunities:
http://waarneming.nl/gebied/info/18398
Hope you enjoy it, birders tend to be pretty actieve in the winter here, lots to see.
Erwin