Vinum, on Lange Gasse, across from the Stiftskirche, has many wines, spirits, and other things, and a great atmosphere to boot. A great place to buy a gift for someone.
Mokka, on Collegiumsgasse near the Wilhelmstift, is a great place to buy teas, coffees, and chocolates. Again, a great place to buy gifts for someone.
There are a some tourist shops that sell souvenirs mugs, T-Shirts etc.: one at Holzmarkt and the other at Marktplatz, furthermore one in tourist information at Neckarbrücke.
University Shop
Neue Aula, Wilhelmstr.
Across the street from the Neue Aula right by the Neue Aula bus stop is a university shop, selling sweaters, mugs, hats, and other things with the University logo printed on it.