The reason I suggested veggie burgers or veggie sausages wasn't because I love them so so much or because I eat them all the time -- they're basically vegetarian 'junk' food when compared with less processed proteins. I suggested them because the grill will already be hot and they're easy to add to the mix and can be served with the condiments and extras the OP would already have for hamburgers. Of course not everyone likes them, but not everyone likes beef either. They're extremely common vegetarian food and easy to find and prep even if you've never eaten a vegetarian meal in your life.
I love grilled mushrooms, but the protein in them is low (2g/cup vs. 11 g for a Morningstar Black Bean burger) and prep might be somewhat unfamiliar or add complication to the already full slate. If the OP is up for that, or any of the other interesting suggestions here (though kebabs plus a platter of roasted veg might be somewhat redundant; one or the other seems great) like making veggie patties from scratch, more power to them!
If the OP doesn't want to scrub down the grill, veggie faux meats can be microwaved or baked, or they can be placed on foil on the grill.
(Then again, many vegetarians aren't so picky about contact with a grill that has had meat touch it -- it's just so hard to avoid in most public scenarios. For me it's a preference rather than an absolute.)
Re: Cheyne's question: I don't eat meat. That doesn't mean I don't eat things shaped like meat or that taste reasonably like it (I do avoid things that are reputed to taste just like beef -- most veggie "burgers" aren't -- because I hated beef when I did eat it). I do eat faux meat in limited quantities because I feel better when I don't eat it in excess! I prefer to flavor my own bean, soy, egg and nut proteins most of the time.