Chill a large pitcher and all liquid ingredients to keep the punch bright and cold.
Gently muddle the raspberries or strawberries in a bowl until slightly crushed to release their juices.
Pour cranberry juice and grapefruit juice into the chilled pitcher and stir to combine.
Add the fresh lemon juice and elderflower liqueur or simple syrup; whisk briefly to blend.
If using alcohol, pour in the vodka or gin and stir gently to incorporate.
Optionally strain the muddled berries through a fine mesh for a clearer punch, or leave them in for texture.
Add ice cubes or an ice block to the pitcher to control dilution as the punch sits.
Just before serving, top the mixture with chilled sparkling water or club soda to add effervescence.
Taste and adjust sweetness with more simple syrup or tartness with more lemon juice if needed.
Serve in coupe or highball glasses, garnished with citrus slices, fresh mint, or edible flowers. Enjoy immediately.