Brussels is small so there's a lot thats easily accessible.
Park + Museums = Parc Cinquintinaire, a large green space with one of those typical European triumphal arches in, either side of which are 2 museums, 1 war, 1 old cars. The war museum is awesome. This isn't that near the station but only 10mins on the Metro away.
Grand Place, is the main old heart of the city, very ornate architecture surrounded by cafes, bars and chocolate shops, and tourist tat, a few stops from Eurostar station (Gare du Midi). Take a south exit from this square and walk to 'Bourse' (the old staock market building), there's some quirky tourist shops around there. Rue Neuve for shopping, pedestrian precinct.
Any Del Haize supermarket should sell you baccy.