My family loves sorghum syrup. It's high in iron, protein, magnesium, and other good stuff, and it even has some fiber, but it's sweet! We use it as our sweetener in oatmeal, coffee, and some baked goods. It's made from a grain that grows in the Midwest; it's hard to find in our area, so we order a big jar online.
Nutritional yeast flakes are yummy and easy to add to any oily food. I'm not sure if they are strictly vegan, though; isn't yeast a simple animal?
We get a can of hemp protein powder whenever it goes on sale. It mixes into spaghetti sauce pretty well. It's good in smoothies. My favorite thing to do with it is put it into cornbread because you expect a slightly gritty texture there, so the grittiness of the hemp powder is no problem, and its flavor improves the cornbread--but it does have a dark green color, so you have to either work with that or add something else with even more color, like pumpkin.
Several kinds of seaweed are very nutritious and sold dried in health food and Asian stores. Sprinkle over rice or baked potato. Add a little to foods that cook for a long time and have complex flavors, like soup or bean dishes or spaghetti sauce, and add less salt than usual; the seaweed will blend right in.
We eat a lot of oatmeal for breakfasts and snacks because it's nutritious itself, you can add a lot of good stuff to it (sorghum, flax meal, nut butter, dried fruit), and it's delicious!
One of my favorite "convenience foods" is kale. I buy a bunch, use some of it fresh, and freeze the rest (finely chopped) in little bags. Then I can add a bag to practically any savory food, without pre-thawing because it thaws really quickly. It's very nutritious and affordable, too!
At nutritiondata.com you can look up lots of details on foods or search for foods that contain specific nutrients.
On my site (click my user name) I have many recipes for the above ingredients. I love figuring out how to sneak in extra nutrition while still making tasty food!