My mom usually makes a simple layered fruit salad with raspberries, blueberries, cut up strawberries, and a sprig of mint on top. Maybe a little glaze of orange juice mixed with honey to drizzle over. It looks really pretty in a bowl that shows the layers.
Also, depending on the food options, a few loaves of french/italian bread can be sliced almost all the way through, then stick a little slice of butter and a bit of minced garlic between them, wrap them in tin foil, and pop them in the oven when you get there.
Another pretty simple option is a pasta salad; cook some penne pasta, half some olives and cherry tomatoes, add some Parmesan, pour some salad dressing (Italian is a good option) you like all over it, then stick it in a gallon plastic bag and bring a big bowl along to dump it in. That one is pretty cheap as well and real yummy if there's leftovers.