Lübecker Marzipan is the most famous export from the City. It is available in several varieties. The excellent Cafe Niederegger (http://www.niederegger.com/) has outlets throughout the city. It's main store on the Alter Markt is a tourist attraction in its own right. It includes a museum dedicated to the Manufacture of Marzipan as well as many Marzipan sculptures.
Walk down the Hüxstrasse and the "Fleischhauerstrase", Lübeck's famous shopping streets and enjoy shopping. You may find artists, painting various interpretations of the Holstentor, which make an excellent souvenir.
Rotspon wine, available in nearly every shop.
Souvenirs and Lübeck-related literature can be purchased eg at the Rathaus bookshop between market place and Marienkirche.