You can use any type of beet, or substitute chopped tomato or more watermelon.
Wrap beet in microwave-safe parchment paper. Microwave at HIGH 3 minutes or until tender. Let stand 5 minutes; thinly slice. Combine sliced beet, watermelon, onion, vinegar, sugar, salt, and pepper in a small bowl.
Spread about 1 teaspoon goat cheese onto each bread slice. Divide beet mixture evenly over bread slices. Drizzle each bread slice with 1/4 teaspoon oil; sprinkle evenly with chives.