Combine the lemon juice, maple syrup, mint, and seltzer in a blender. Blend for 1 minute, or until the mint is totally blended in. Add the ice cubes through the hole in the lid, one at a time while the blender is running.
Serve with fresh lemon slices, and mint.
To make this a cocktail, add a double shot of tequila or vodka to the blender.