Tomatoes are mildly acidic, so something similar should get you an approximation of the flavor. Pickled onions might be a good choice, or some other lightly pickled fruit/veg (maybe beets for salad). Actual dill pickles are not going to be too acidic to be a good substitute (but are a great thing to put on sandwiches in addition to tomato anyway).
Pineapple might work in some cases, but it has a strong flavor and sweetness that won’t work everywhere a tomato does. Salsas are a good place for this as a tomato substitute (along with mango and corn), but pineapple does also work well in some sandwiches.
Tomatoes are mildly acidic, so something similar should get you an approximation of the flavor. Pickled onions might be a good choice, or some other lightly pickled fruit/veg (maybe beets for salad). Actual dill pickles are not going to be too acidic to be a good substitute (but are a great thing to put on sandwiches in addition to tomato anyway).
Pineapple might work in some cases, but it has a strong flavor and sweetness that won’t work everywhere a tomato does. Salsas are a good place for this as a tomato substitute (along with mango and corn), but pineapple does also work well in some sandwiches.