Ahi: Rinse 3 ounces sashimi-grade ahi tuna (yellowfin or bigeye; for bacteria safety, buy it from a good seafood purveyor); pat dry. Cut tuna into 1/4-inch-thick strips about 1/2 inch wide and 3 1/2 inches long. Just before serving, arrange raw fish on a bed of crushed ice on a rimmed plate.
Caviar: Place 2 ounces salmon roe in a wire strainer and rinse gently under cold running water; mound in a small bowl.
Crab: In a small bowl, mix 8 ounces shelled cooked crab or drained canned crab (squeeze out liquid) with 1/4 cup mayonnaise and soy sauce to taste (about 2 teaspoons). Makes about 1 cup.
Smoked salmon: Cut 3 ounces thinly sliced smoked salmon into 1/2-inch-wide strips.
Asparagus: In a 5- to 6-quart pan over high heat, bring about 1 quart water to a boil. Snap off and discard tough stem ends from 8 ounces asparagus. Rinse asparagus and add to boiling water; cook until barely tender when pierced, 3 to 4 minutes. Drain and immediately immerse in a bowl of ice water. When cool, lift out and cut into 3 1/2-inch lengths.
Avocado: Pit and peel 1 firm-ripe avocado (8 oz.); slice lengthwise into 1/4-inch-thick, 1/2-inch-wide strips about 3 1/2 inches long. Place in a bowl with 2 tablespoons lemon juice; turn to coat.
Carrot: Peel and shred 3 ounces carrot, or use 3/4 cup shredded carrot.
Cucumber: Rinse 6 ounces Japanese or English cucumber; cut into 3 1/2-inch lengths. Cut lengthwise into 1/4-inch-thick slices, then stack 2 or 3 slices at a time and cut lengthwise into 1/4-inch-thick sticks.
Mushrooms: Trim woody stem ends from 3 ounces enoki mushrooms. Rinse mushrooms briefly, drain, and gently pat dry.
Spinach: In a 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil. Add 6 ounces rinsed spinach leaves and cook just until wilted, 1 to 2 minutes. Drain and immerse in a large bowl of ice water until cool. Drain again and spread on a clean kitchen towel. Roll towel up and twist and squeeze tightly to remove as much liquid as possible.
Sprouts: Gently rinse and drain 2 oz. (1 1/2 cups) radish or broccoli sprouts; trim off root ends, if present.
You can get the fillings ready up to 4 hours ahead; chill airtight until you're ready to roll.