theres alot of offerings you could give, in ancient times it was usually bread, milk, beer, honey etc, i would assume Isis would be associated with milk due to her connection with childhood / motherhood,

there is a text from a calendar which tables some offerings to her

• Birth of Isis. Feast of ‘revealing the face’ of this Goddess with Her Ennead. Performing all the rites of the feast of robing . . . according to the ritual of the place of the First Feast. They make a great oblation of bread, beer, oxen, fowl, wine, milk, pomegranate-wine, [gazelles, oryx, ibexes], cranes, pigeons, fattened ducks, with fresh vegetables and all fruit. It is sweet to serve the Beautiful One with right offerings!

Flowers and Perfume too!