This is only my first attempt for the evening, but I tried it and went full stop - this is fantastic, it's getting posted. It's a little more complicated than some of my attempts from last night, but it works marvelously together - a little sweet, a little sour, a bit bitter, expressive of watermelon and summer, and very refreshing.
1 1/2 oz cryo-watermelon juice (a great way to use up all your leftover watermelon)
1 oz strawberry-saffron syrup (the saffron adds a certain deep, herbal quality that is entirely worth the expense, believe me)
1/2 oz lime juice
3 dashes rhubarb bitters
Shake well and strain over large cubes in a Collins-style glass. Top with about 2 oz each tonic (Q Tonic for me) and carbonated cucumber water (don't use regular soda, the cucumber really brings out the watermelon flavor).
I'm gonna go have another.