Salads are a great way to obtain nutrients and fiber while on the ketogenic diet. Lettuce is generally low in carbohydrates (per serving); however, I recommend sticking to spinach and/or arugula based salads since they have the most fiber and least net carbs. Top your salad with keto-friendly veggies such as radishes, mushrooms, broccoli raab, etc. Even a little bit of bell pepper is ok to add to your salad. Avoid "sweet" veggies or fruits and definitely skip the croutons. The main thing to keep in mind is to not overdo it by eating a large salad. A nice pile of healthy veggies over lettuce can quickly add up to 20-30 carbohydrates. Be sure to add fat to your salad in the form of eggs, avocado, or some kind of cooked meat.
Another important thing to remember with a salad is to choose a ketogenic friendly dressing. Choosing the wrong dressing can quickly ruin your diet. Many salad dressings are full of sugars. Stick with oil/vinegar based dressings (check the labels to avoid added sugar), or make your own at home. You can even make homemade ranch dressing out of a ranch seasoning packet and greek yogurt. Another great choice can be avocado based dressings. Plain balsamic vinegar is one of my top choices.