You can add all kinds of other vegetables to your salads depending on your mood. I generally prefer simpler salads that allow one or two ingredients to really shine, but to each his own. Tomatoes and onions seem to be standard fare but don't forget cucumbers, shredded carrots, shredded cabbage, avocado, olives, celery, cauliflower, broccoli, radishes, peas, corn, beans, etc. Lightly steamed veggies that have been chilled also make excellent salad additions, think asparagus, green beans, broccoli or cauliflower. While raw mushrooms are a staple of many salad recipes, I now avoid them.
Fruit can add a bright flavor surprise to green salads.
For just a few suggestions, try adding a small amount of sliced fresh strawberries, blueberries or raspberries to your salad mix. Citrus fruit like orange or grapefruit slices or fresh pineapple go really well, especially with dark greens like spinach. Add tropical flavor with fruits like mango or papaya or my favorite. Add grilled chicken, beef or even tofu to your salad and turn it into a meal. If you like toasted nuts on your salad, try sugared nuts, such as pecans or walnuts, for an even greater flavor spectrum.