2 to 3 tablespoons agave nectar
Sparkling mineral water, or seltzer
1) Juice lemon into a bowl and strain out seeds. Add agave nectar, to taste, with juice and blend. Pour into glass with ice and top with sparkling water.
You can also try: cutting down on the agave nectar and replacing it with some sweet, concentrated juice like black cherry or pomegranate. Or keep all the agave and splash in some strawberry or cranberry juice (and don't forget to add some frozen pieces of fruit, as well). Mint can also be tasty.
If making this for several people, it's nice to serve in a pitcher. I suggest freezing the lemon juice/agave mixture in ice cube trays to add after you have made some of the full lemonade recipe. This helps keep the lemonade from becoming watery (especially not-so-good when the aim is for it to be carbonated). To keep it nice and cold, you can also slice and freeze lemon rounds. Enjoy on a sweltering summer day.