*Edited I've just realized that sims with different traits will react differently to some of the toppings/garnishes! It's quite cool. Need more game time to figure out exactly which trait has a 'secret' reaction but a sim with the insane trait is interesting when they eat an ice cream with nuts. lol Changed the text re: toppings above to reflect this hidden combination thing.
For reference, I extracted from the tuning files the combinations of garnishes and toppings that give a buff:
(Order: Garnish, then Topping)
Childish or Child: Sprinkles, Whipped Cream.
Slob: Sprinkles, Caramel Syrup.
Lazy: Sprinkles, Sprinkles.
Materialistic: Leaves, Chocolate Sause.
Creative: Leaves, Raspberry Sauce.
Neat: Leaves, Whipped Cream.
Romantic: Leaves, Cherry.
Active: Strawberry, Chocolate Sause.
Good: Strawberry, Whippped Cream.
Art Lover: Orange Slice, Raspberry Sauce.
Outgoing: Orange Slice, Cherry.
Genius: Orange Slice, Coconut Shavings.
Bro: Pepperming Stick, Chocolate Sause.
Hot Headed: Peppermint Stick, Whipped Cream.
Commitment Issues: Peppermint Stick, Nuts.
Foodie: Flower, Raspberry Sauce.
Clumsy: Flower, Sprinkles.
Loves Outdoors: Flower, Coconut Shavings.
Glutton: Waffle Piece, Chocolate Sause
Perfectionist: Waffle Piece, Caramel Syrup.
Geek: Waffle Piece, Sprinkles.
Evil: Cinnamon Stick, Raspberry Sauce.
Snob: Cinnamon Stick, Nuts.
Self Assured: Cinnamon Stick, Coconut Shavings.
Music Lover: Banana Slices, Chocolate Sause.
Goofball: Banana Slices, Whipped Cream.
Mean: Nuts, Caramel Syrup.
Gloomy: none.
Insane: Nuts, Nuts.