My DD love tinned tuna mixed with mayo, corn, grated carrot and chopped celery. She will eat it as a salad sort of thing, or on a sandwich or with crackers to 'dip' into it. Or sometimes I cook baby potatoes and make it into a tuna potato salad, which I love too!
How about homemade fish fingers. We do this for dinner, but it would be just as good for lunch.
My DH loves prawn sandwiches- small fresh prawns mixed with shredded lettuce and seafood sauce in a sandwich/ wrap/ roll.
Seafood sushi.
Tuna/salmon mini quiches.
DD also loves sardines mashed up with a little salt, very finely chopped spring onions/onion and a dash of vinegar on toast.
Tinned tuna/salmon on a baked potato.