"You have tasty sandwiches."
Translation:Wy macie smaczne kanapki.
In this case yes. For full declension see e.g.
(cases from top to bottom are nominative, genitive, dative, accusative, instrumental, locative and vocative)
"Ty masz kanapki smaczne"? That sounds as if "tasty" was a category, a type of sandwiches, not a simple description. It's really weird.
Compare with "Jem zupę pomidorową" (I am eating tomato soup), where the adjective "pomidorową" is after the noun "zupę", because it's a kind of soup.
"kanapka" is feminine, yes.
"kanapki" is plural, and we don't really talk about masculine/feminine/neuter in plural. There's 'masculine personal' (for plurals of masculine nouns describing people) and 'not masculine-personal' (for any other plural nouns). "kanapki" belongs to the second one.
It's just that 'not masculine-personal plural' and 'neuter singular' have the basic forms of the adjectives identical (e.g. "smaczne jabłko" = a tasty apple, "smaczne jabłka" = tasty apples).