I first had this served to me by a lovely, grandmotherly coworker. I believe she said she got the idea from Martha Stewart, but I might be misremembering. Anyway, this recipe is available many places online, but that's not going to stop me from putting it here.
Simple Syrup Herb Soda
Basically, you make a simple syrup, which is a one-to-one ratio between sugar and water. Let's say a cup of each. Bring that to a boil, and toss in, oh, a third of a cup or so of fresh herbs that you've crushed a bit to bring out the oils. So far basil is my favorite. I've also tried lavender (a little too floral/sweet, but I think it would be awesome if you also mixed in some lemon zest) and lemon verbena (YUM!). Anyway, turn off the heat under the water, cover, and let steep for 5-10 minutes depending on how strong you want the herb flavor to be. (I usually go 10.) Chill.
To serve the sodas, put some ice in a glass, fill about 1/5 to 1/4 with the syrup, then top off with club soda. If you want to be fancy, you could garnish with a sprig of whatever herb you used.
This is a fantastically refreshing summer beverage, and there's lots of flexibility around creating it to taste.