Oh, dearie me. All of these ideas are great. I slice my zucchini and eggplant (courgettes and aubergines) lengthwise, very thickly. Brush with olive oil and season with my favorite garlic blend salt. I grew up eating onions, sliced and with a cube of butter (preferably frozen) and S&P, wrapped in foil. Various other veggies really take to these treatments. I do them as a side to the meat, but in fact, when we were on the road, I'd do the veggies first, and old guys who I think would be apalled at the word "vegetarian" would stop and ask me what I was cooking. The meat was still in the fridge, marinading, it was the aroma of the veggies that got everyone in the campground's salivary glands working overtime. To turn potatoes into a good starch, nuke them until hot, but not quite cooked (this is impossible to predict, microwave ovens are very different in power). Then either put in foil with onions and S&P, or if small reds, just roll in a little olive oil, then put on the grill until browned and tender. Winter squashes: Pierce several times after washing, then grill until almost black on all sides. Remove when almost black (dark brown) all around. put on the counter while you deal with the rest of the meal. Then halve, scoop out seeds, then scoop the flesh into a bowl. Add what you like (I personally like butter or olive oil, and either garlic seasoning or just S&P, but many would toss in a spoon of brown sugar, maple syrup, or a touch of molassas. I find winter squash sweet enough as it is.) Spaghetti squash really works well this way, just treat it as you would any pasta.
Big mushrooms are great grilled on one side, with some cheese, stuffing mix, or whatever filling it. Grill on the hollow side, turn over, and spoon in the stuffing. Both portabellas and large regular mushrooms do well with this treatment.