Instrumental form of "kanapki" is "kanapkami". But of course the sentence founds funny. On the other hand other uses of instrumental- with="using", and after prepositions, most often "z"=with
the plate with sandwiches= talerz z kanapkami
the box filled with sandwiches= pudełko wypełnione kanapkami
I still have a lot of problems with Polish cases, but the declension for ''kobieta'' can be found here.
You can find the declensions for any Polish noun by looking it up on Wiktionary.
You can also find the conjugation of many verbs the same way.