On Saturday early morning you may not meet too many people in the streets of Prague but three places will be overcrowded for sure: Vítězné náměstí, "Jiřák" and Náplavka riverside. That's the locals who got up early to be the first ones to grab fresh vegetables, free-range eggs or bread from local sources, often organic or environmetally friendly.
There are lots of things to be tasted directly, some of them copying traditional old Czech recipes while others might introduce international or alternative cuisines. It is also a great place to buy a bottle of local wine or cider, sometimes handicrafts are available, too. P.S. Jiřák is open also on Wednesdays and Fridays.
Hard to pick one :-). Try some Slovak cheese, hand-made potato crisps and definitely some sweets. Often there is a live music, don't be shy to dance!
Mon-Fr: 09:00-20:00