"You have tasty sandwiches."
Translation:Wy macie smaczne kanapki.
Is this smaczne kanapki really in the nominative case with this scentence structure?
In this case yes. For full declension see e.g.
(cases from top to bottom are nominative, genitive, dative, accusative, instrumental, locative and vocative)
jesteście= you (plural) are jesteś= you (singular) are macie= you (plural) have masz=you (singular) have
wy (you plural) and ty (you singular) are usually optional and not included unless emphasis is wanted.