Opening hours in Toulouse are generally Mon-Sat 9AM-1PM and 3PM-7PM, but there are numerous exceptions.
souvenirs
As Toulouse is a city of aviation and spaceflight, check Airbus and the Cité de l'Espace for souvenirs
flea market
There's a flea market every Sunday morning near the Basilique Saint Sernin. While it does not offer anything too special as flea markets go it's a great way to mingle with a local crowd. Another flea market is held every first weekend of the month at the Grand Rond
Also a very nice market around the St Aubin Basilic every Sunday morning, selling producer's vegetables and fruits. Very relaxed atmosphere.
Every weekday there is a vegetable market before noon along the Boulevard de Strasbourg - this is actually the cheapest of all the vegetable and fruit markets of the city
There are excellent 2nd hand shops or "friperie", especially on Rue Gambetta and in the whole quarter behind the 'Ecole des Beaux Arts', around the 'place de la Bourse' which is the historical textile production quarter of Toulouse
If you are on a budget, the supermarkets where most students use to go are the brands 'Lidl' and 'Leader Price'. Those are to be found more on the 'edges' of the city though some are accessible with the underground. In the centre, you'd rather go for Champion or Géant Casino.