I have to recommend going to Lakeside beach. You are only about a 5-10 minute drive. It is my absolute favourite beach on PEI. You will be able to find lots of shells there. As for the sea glass I would recommend going a bit further east to the Souris area! You never have trouble finding sea glass there. Souris is about a 30 minute drive from Mount Stewart. I would recommend checking out Sally’s beach.
I hope you have the best time in PEI!