Whatever meats and veggies you like. There are no rules. Onions, green peppers, cherry tomatoes, apple slices, pineapple cubes....
My tip for you is this:
Although it looks "cool" to arrange assortments of veggies and meat on each skewer, they will cook better if you put just meat and just veggies on the skewers. That way if one thing finishes first, you don't have to overcook everything else on the skewer.
For presentation purposes, you can always reskewer them, but I usually pile everything on a serving plate and go from there.