It depends, to me, on whether or not you will be preparing your dish on site or whether you need to take it with you, keeping it warm somehow.
If you can prepare on site, you could do a platter of various steamed veggies, sprinkled with salt, pepper and a little parmesan cheese. Something that gives you a wide range of colours. My Mom always did a platter of cauliflower, carrots, broccoli, asparagus and brussel sprouts. You could to this same kind of thing with the roasted vegetables, like Zilla described. Or if the in-laws have a grill, you could do a variety of grilled vegetables quickly. Aspargus, zucchini, peppers - brush with olive oil, toss with salt and pepper and grill either directly on the grill or in a grilling basket.
But if you need to take the dish with you already prepared, I'd go with something more casserole like as it will keep warm better and deal with reheating better. A friend of mine does a broccoli, cauliflower and other veggies in a cheesy sauce, with a bread crumb topping.