This is the 10982nd most frequent Polish word.
fantastycznych
"Fantastycznych" means "fantastic" or "fantastical" in English, typically as a plural adjective.
In this sentence, "fantastycznych" is used as an adjective in the genitive plural form, describing "przedmiotów" (items). It conveys admiration or an extraordinary nature.
Odwiedziłem wczoraj sklep pełen fantastycznych przedmiotów.
Yesterday, I visited a store full of fantastic items.
Here, "fantastycznych" is an adjective in the genitive plural form, modifying "atrakcji" (attractions), indicating impressive or remarkable features.
Zorganizowaliśmy imprezę z wieloma fantastycznych atrakcji.
We organized a party with many fantastic attractions.
In this sentence, "fantastycznych" is the adjective in the locative plural form applied to "światach" (worlds), referring to imaginative or extraordinary characteristics of those worlds.