Light, Chilled Salads!!!
How about a 3 bean salad with a light oil and vinegar dressing? Super easy, just open 3 cans of different beans, rinse (or in your case of a HUGE party, 3 cans of 3 different kinds of beans!!) Oil and vinegar to top, thin sliced onion and a bit of garlic minced. Some people add a bit of finely chopped green pepper.
or
A tomatoe, red onion, mozzarella salad....slice the 3 into uniformly thinnish slices, drizzle with a light italian dressing.
or
Chilled brown rice veggie salad. Cook enough rice to have 3-4 cups cooked, chill. Par-boil or blanch carrots and brocolli...(litely cook, just to brighten the color and soften a tiny bit) chop them finely. Chop the tops of a bunch of green onions, cut in half and squeeze out the seeds and juice of 2-3 roma tomatoes, dice, add a tsp of chopped/minced garlic (about 1-2 cloves). Toss is all together and dress with a light oil and vinegar dressing. Let it sit over night and then top with a bit more of the dressing as the rice absorbs alot.
Both are best if made the day before to let the flavors really intensify.
You could also grill some veggies. I've not done that myself but have had them at bbq's and they are great.