A topping is just an ingredient added in a smaller quantity, to contribute more delicious flavor, texture and nutrition. The idea is for the pieces to be small and scattered enough that they don’t dominate any mouthful.
Here are a few ideas to start with:
Seeds & nuts: cashew, pistachio, almond, pecan, walnut, hazelnut, macadamia, peanut, sesame, (ground) flax, pumpkin, sunflower, hemp hearts.
Sweet: dried cranberry, raisins, chopped up dates
Crunchy: croutons (dried bread), tortilla chips, grated or thinly sliced or chopped up hard veggies (carrot, beet, radish).
Tangy: pickled or fermented veggies like cucumber, beets, beans.
Fresh herbs: Chopped up herbs like basil, parsley, dill, cilantro, tarragon, thyme, any fresh herb can work. They can go straight into the mix, or try them in the dressing.
Grated: ginger, horseradish, Jerusalem artichoke, finely grated sharp cheese..
Try lots of different toppings, and keep a stock of your favorites always on hand.