1. Bring sugar, lime zest and juice, cardamom, and salt to simmer in small saucepan over medium heat, stirring constantly, until sugar is dissolved, 1 to 2 minutes. Remove pan from heat, stir in cucumber, and let syrup cool for 20 minutes.

2. Meanwhile, cut away peel and pith from grapefruit. Slice into 1/2-inch-thick rounds, then cut rounds into 1/2-inch pieces. Place grapefruit and pineapple in large bowl.

3. When syrup is cool, pour over grapefruit and pineapple and toss to combine. Refrigerate for 15 minutes before serving.

Serves 4 to 6