I'm a comfirmed omnivore, but often have had to feed various types of vegetarians and a huge suggestion (if I missed it in reading, pardon) is make a cookout. Take a wide variety of veggies (all kinds of squash, eggplant, onions (again of all types), large mushrooms -- well, anything. Cut them large enough that they don't fall through the grill (even with a basket if too small they'll burn too fast if you aren't careful. A zucchini, for example, just gets cut in half lengthwise). Toss in olive oil and your favorite seasoning blend (mine's Cavendar's Greek for this purpose), then grill until the degree of softness you like. Chop into eating size and serve.
I guarantee that if you bring the veggies to a cookout, you'll gather a crowd (bring them raw and just get in there and cook them while the meat-eaters are doing the steaks. If it is burgers and dogs, you want to get in there first). The great thing is that you can do the veggies first, put in a bowl, cover w/foil, and let the meat cookers have at it. Yours will be ready when theirs is. In my camping days those older guys who didn't "do" vegetables and didn't "get" vegetarians would stop at my weber and ask what I was cooking that smelled so good.
Leftovers are great in a salad or on pasta or rice.
Potatoes (sweet or otherwise) and winter squashes are best "softened up" a bit in the microwave oven before grilled (take too long sometimes otherwise). Sliced potatoes and onions, butter/margarine/olive oil, S&P in foil make a dish I was practically raised with. Spaghetti squash: Pierce several times. Place on grill. When brown on all sides, put aside to cool a bit. Cut in half and fork out the innards. Top with anything you like. Nice, smokey flavor.