Lentils. You can buy them dried but they’re also available already cooked in cans and pouches. They are cheap, highly versatile, nutritious, and cook quickly. Use in soups and stews; as a ground meat substitute in tacos, nachos, veg burgers, shepherds pie; in salads and wraps; mixed with brown rice for a complete protein. Lots of great ideas here.
https://www.self.com/gallery/lentil-recipes
Lettuce doesn’t last long but if you’re thinking of a salad to get veggies into your daily diet try a salad of veggies that last: shredded carrots, red cabbage, chopped cucumber or zucchini, cherry tomatoes, a boiled egg, maybe some brown rice.. drizzled with a tahini dressing for healthy fat (tahini, mustard, lemon juice, olive oil, salt and pepper).
If you want to eat healthier but don’t know much about nutrition, start with ‘eating the rainbow’. Get as many different colored veggies into your diet as possible.