Just about any vegetable or fruit can make a worth salad addition. Raw is the obvious first choice, for ease and nutritional value. But cooked can work as well. Imagine tossing in a handful of cooked black or kidney beans, that sounds good, too.
Raw: To get you started, consider regular and cherry tomatoes, zucchini, cucumber, radish, broccoli, cauliflower, snap beans, cabbage, kohlrabi, peas, snap beans (green, yellow, purple), summer turnip, onion and green onion, grated beets and carrots, grapes, orange segments, melon, apple, pear, cherries, berries.
Cooked: Hot or cold, whole or cut up, there are lots of cooked possibilities, like beans, potato or any other root crop (beet, carrot, even parsnip or rutabaga, why not?).
Size & shape can make a big difference in appearance and taste. You can cut up quite small, or into bigger chunks or strips. For raw, hard veg and fruit, consider using a vegetable peeler, grater, or gadgets like the spiralizer. How you cut determines what fits in a mouthful, which in turn determines how your salad tastes.
Your only concern with what to add is keeping your salad from getting unfun to eat. For example, banana slices may fit in perfectly, but if they quickly become mushy or discolored, that’s probably not fun. It’s up to you. Try it all!