I second the suggestion for freezing brownies. Un-iced cakes can usually be frozen. You could make and freeze a retangular banana cake for example, and the night before take it out of the freezer and put into fridge to defrost overnight. In the morning, ice it with nice passionfruit icing, cut into squares and arrange on a platter. Or freeze plain vanilla cupcakes and make fairy cakes.
I have never done this but apparently scones can be frozen. Scones with jam and cream are always popular at an afternoon tea.
Not sure if you are having any savoury stuff but mini quiches, sausage rolls and spinach triangles freeze well.
I always make the sandwich fillings in advance and have them sitting ready in the fridge. Then it only takes a few minutes to assemble to sandwiches. You can also boil eggs the day before. I make up chopped hardboiled egg with mayonaise, chopped chives and lots of salt and pepper or curry powder. You could buy a bbq chicken the day before, and in the morning chop up the chicken meat with mayo, herbs, salt and pepper. Could also add finely chopped walnuts or pine nuts if no one is allergic.
Lots of slices can be made in advance and keep well in an airtight container for a few days. Rocky road is easy to make and keeps well.
Good luck, hope you have a nice party.