A peanut butter or almond butter and jam sandwich could easily have 500 calories if you put enough nut butter on it.
Pita and hummus is another option, you just might need a fairly big portion of hummus.
A marinated/baked tofu sandwich (they sell already marinated and baked tofu in a lot of stores, at least in the US, where I live).
Rice and beans is something you could cook at night and then pack for lunch the next morning. You could also put it in a tortilla with some tomatoes and lettuce for an easy burrito. Even without the rice, it would still be good (if you like burritos).
Soup with a couple of slices of whole grain bread. It's not great to have canned stuff all the time, but in a pinch, it's an easy option. Alternately, you could make a pot of soup on a day off and pack leftovers for lunch.
Pretzels and hummus - an old favorite easy meal for me.
Leftover spaghetti and sauce, which you could put some nutritional yeast on for extra nutrients and flavor.
A big salad with tofu and/or hummus on it, with crackers or bread to add to the meal.
Good luck on your vegan journey!