You don't eat 'vegetables' for the calories - you usually eat them for the vitamins, minerals, anti-oxidants, fibre, water and phytonutrients they will offer you...
Most vegetables are 'good vegetables' with things like celery, lettuce, cucumber, green beans, broccoli, cauliflower, cabbage, brussels sprouts, tomato, eggplant, capsicum/peppers, mushroom, spinach, kale, rocket, zucchini, asparagus, leeks, bok choy, chinese choi sum all being great choices...
The only ones you have to be a little careful with are things like green peas (a legume not a vege - so it is more calorie dense), corn (a grain like oats), carrot (more sugar than regular vegetables), beetroot (more sugars than regular vegetables), squash/pumpkin (more calorie dense and less fibre) and parsnips (more calorie dense)...
But other things you can add to salads are legumes (beans, lentils, chick-peas), fruits (strawberries go really well), seeds or nuts (sunflower seeds, walnuts), avocado or olive oil (for the healthy fats) or another carb such as barley, sweet potato, brown rice or even sweet corn.