Make your own grape juice by pureeing and straining fresh grapes for this sweet and tangy sparkling drink. If you have very sweet grapes, you might not need the full amount of honey. Taste as you go. For a special treat, freeze extra whole grapes ahead of time and float them in the drink along with the ice cubes.
Puree grapes in a blender. Pour the puree through a fine-mesh strainer set over a medium bowl or large measuring cup. Press on the solids to extract all the juice. Add lemon and honey to the juice; stir until the honey is dissolved. Add seltzer. Divide among 4 ice-filled glasses. Garnish with whole grapes.