Fantastic and full of junk - everything from liquor to peruvian blankets and faux Nikes. No antiques, little that is unusual or one of a kind, nothing worth going out of your way for unless, like me, you enjoy junky crowded markets. I don't go to shop, just to watch. Perhaps during tourist season there's more on offer?
Take the bus to the market then walk around the neighborhood. There are some excellent coffee shops -- meaning purveyors of deep black coffee that is gritty and a shock to the system - not restaurants.
The market is every Tuesday I think... also, the market is nowhere near the 'old town' bit of Sintra. Don't try to walk it unless you've got an excellent map.