Some classic dessert garnishes:
Sauces (cold or warm): chocolate, vanilla custard (crème Anglaise), caramel
Fruit purées: raspberry, strawberry, blackberry, mango, kiwi, peach
Fresh fruit: (sliced or whole) raspberries, strawberries, blackberries, blueberries, pomegranate seeds, mangoes, kiwi, peaches, star fruit, pineapple; caramelized banana and/or pineapple
Citrus: fresh or candied zest, fresh or candied rounds
Edible flowers: pansies, rose petals, marigolds, tulip petals, orchids, violets, nasturtiums, orange blossoms, snapdragons
Herbs: mint leaves, lavender sprigs, lemon thyme, rosemary
Powders: confectioners' sugar, cocoa powder, ground cinnamon, ground nutmeg
Ice cream: any flavor; if you make your own, try a sour cream or crème fraîche ice cream (substitute sour cream for half of the heavy cream) for a sweet-tart contrast.
Whipped cream: plain, sweetened, or enhanced with liqueur or flavored extracts. Try whipping half heavy cream, half crème fraîche with a little sugar for a tangy-sweet topping.
Chocolate: shavings, curls or other chocolate designs
Nuts: whole or chopped, plain or candied, toasted or raw (ground raw pistachios are a beautiful garnish); long shreds of fresh or toasted dried coconut
Cookies: any kind, but tuiles and butter cookies are the most versatile