Tomato slices, bell pepper, sliced onion and capers - this is exactly what I put on homemade mini-pizzas (I get an accidentally vegan pizza sauce and put it on whole grain toasted English Muffins and use either Daiya or Follow Your Heart shreds for the cheez to melt on top of my veggies!)
I highly recommend this but if you are in a restaurant which doesn't have capers, I would default to black olives instead.
There are chain restaurants like Pizza Rev or Mod pizza in parts of the U.S. where you can get vegan "sausage" on your pizza, too. I used to go to a place in L.A. I miss so much - it's called 800 Degrees. Their crust and sauce is so freaking gourmet and perfect, that I would just order the pizza with olive oil, crushed garlic, tomato sauce, Daiya shreds, and fresh torn basil on top. I don't recommend that with most chain restaurants or cheap pizzas though.