Buffet style food?
Homemade dips
Veg trays
Salad bowls
Fruit bowls
Quiches
Cheese and pineapple sticks
Sugar free jelly pots (lighter cream at side)
Wholemeal pasta salad
Cold new potatoes
Wafer ham pieces rolled up
Spiced chicken pieces
Picketed onions
Yogurt pots
If not everyone is dieting I'd still make things like:
Sandwiches
Crisps
Cake
Nuts
Also some people although dieting like a treat day so provide good choices and let them decided how much of other stuff they want.