6cupsvegetables cut into 1 ½-inch pieces(such as red onions, bell peppers, zucchini, summer squash, whole cherry tomatoes, mushrooms, eggplant, or lightly cooked cauliflower or broccoli; see Note)
Soak about 20 6-inch wooden skewers, or 10 12-inch skewers, in enough water to cover for 30 minutes. You can also use metal skewers, which do not need soaking.
Combine the lemon or lime juice, garlic, olive oil, herbs of choice, salt, and pepper in a small bowl or plastic container and mix well.
Place the vegetables in a mixing bowl, then pour over the marinade and toss to combine. Marinate for 30 minutes at room temperature, or refrigerate, covered, for at least 2 hours and up to a day.
Thread the skewers with the veggies, creating appealing patterns. Lightly salt and pepper the skewers.
Heat the grill to medium-high. Grill the kebabs 4 inches from the heat source, until the vegetables are cooked through, and everything has a nice grill-marked exterior; anywhere from 8 to 15 minutes, depending on which vegetable you choose. Brush the kebabs with some of the marinade (if using) left behind in the bowl as they grill.
Serve hot or at room temperature.
Notes
If you prefer to just use olive oil, salt, and pepper instead of adding lemon juice and garlic to the marinade, that's fine! You can skip any marinating time.
If you are looking to grill more dense vegetables, like potatoes, cauliflower, broccoli, winter squash, and the like, it is usually smart to par-cook them before grilling them, meaning to cook them part way through. Because they are so hard, they will take a very long time to become tender on the grill, and because of that, you will find it a little too easy to char the outsides before the insides become soft. And, if you are cooking other vegetables or meat on a skewer with these denser vegetables, those other items will cook through much more quickly, and you will end up with some overcooked items (mushy tomatoes) while you wait for the broccoli to become tender.
Try steaming harder vegetables until they are somewhat tender but still firm in the middle before skewering and grilling them. You can cube them before steaming or boiling them and then let them cool before skewering them up. This might take between 5 and 15 minutes, depending on how hard the vegetable is.
Make sure the vegetables are nicely oiled or coated with the marinade.
Close the lid of the grill in between turning the kebabs to allow the heat to build and the kebabs to cook faster and more evenly.
Keep your eye on the skewers as they grill, and turn them as they brown on the bottoms. Remove each kebab as the vegetables become tender and the edges of the vegetables start to brown.