Just choose say 2 sweet and 2 savoury thing and make plenty of them. You don't need endless variety.

Scones with jam and cream? Slice? Biscuits? Sushi platter (provided you can keep it cool right until serving), wraps/sandwiches? Don't have anything that needs heating.

Why not just get the pizzas at your place? Heaps cheaper (smaller group) and means you don't have to cater. Of if you do, perhaps burgers (buy the patties) - just need bread rolls, sauce, cheese, salad stuff and meat.

I hope it's a lovely farewell xo