I've been to the cafe once this past summer and was pleasantly surprised by the quality and type of food they were offering on their menu.
It's not fine dining but it's the best the island has to offer away from the them-park type of food the main attractions bring in. The rectory Cafe is tucked away in the residential area of the islands where a quiet cottage feel takes over once you head eastward from Centre Island. The patio is a lovely space with comfortable seating and views of the lake.
The menu offers anything from light bites like salads apps and sandwiches to brunch omlettes and hash to savoury dinner curries, steaks, pasta etc.
We shared the octopus appetizer which was delicious and tender with a perfect char on it. The jicama salad it came with was very refreshing. We also shared a chicken sandwich which was fresh and flavourful.
The prices are a touch steep for a cafe but still the quality is there and if you're willing to spend a nice afternoon on the island in the summer I would recommend you do it at the Rectory Cafe.