What limitations do you have (ie are you limited to cold buffet or is there somewhere to cook food at the venue?)
I'd personally add in coleslaw, potato salad, pesto pasta salad, various dips (humus, salsa, tzatziki, guacamole) and bowls of nachos for dipping.
You need something substantial so either sandwiches or pizza.
I'd also do some chicken wings and some grilled halloumi slices.
Fruit platters (grapes, all the berries, kiwis etc) and salad platters (diced peppers, cucumber, cherry tomatoes, avocado, carrot batons - may not all get eaten but relatively inexpensive and are there for those who want them)
Bowls of sweet stuff (one bowl of marshmallows, one bowl of sweet and salted popcorn, one bowl M&M's, one bowl gummy bears - that kind of thing)
Obviously you need to cater for allergies and intolerances.