Following the advice of SFizzle, here is a salad that can really have anything and tuna in it.
The one we made had tuna, lots of sweetcorn (very popular around here), tomatoes, cucumber, lettuce, olive oil, apple cider vinegar, salt, pepper and smoked paprika.
For more adventurous palates, I add very finely chopped onions, lentils, chives or even red rice.
If using onion, squeeze some lemon or vinegar over the chopped onion to lightly cook it before adding it into the salad.
One could add diced (cooked) potato, dice peppers, diced celery, or you could make it a chicken salad and replace the tuna with some shredded chicken breast.
Warning: if you think you can trick anyone by sneaking the tiniest pieces of disliked food into the salad, think again. It will be carefully picked out and left on the side.