Port Stanley options?
We always enjoy the Pineapple Dining Room in the Telegraph House.
I was there last weekend and had a great platter of perch and a glass of wine on the porch at Mickey's Boathouse. A little more causual...a little more of what you'd expect from a beach/port town.
Not sure if you go there often, but if this is a first visit, you should probably have fries at Mackies (as someone else posted) and try there unique, orange fry sauce.
I enjoy ME and Suzie's. It is pretty casual, it has a nice patio, and the food that I have had was quite good. The emphasis is on local-grown as well.
If you want really, really, really casual you can't beat Mackie's fries, located right on the beach!