Here's a good (and somewhat rare for me!) non-alcoholic recipe. It's essentially my attempt to recreate San Pellegrino Pompelmo (an actual grapefruit soda, unlike my ironically-titled Grownup version) with some enhancements to just pretty much every citrus-related ingredient I had at hand.
6 oz lemon syrup (a standard simple made with extensive lemon zest, one of my favorites)
6 oz fresh ruby red grapefruit juice
2 oz fresh lime juice
20 oz soda water
6 dashes orange bitters
6 drops orange flower water
Combine the above in a 1-liter glass bottle.
This is tasty just served over ice as a refresher. Of course, if you want something boozy, I find that this makes a delicious gimlet/tonic hybrid if combined with dry gin (or vodka, or light rum, or white tequila) in about a 2:1 (soda : liquor) ratio. I also like to spike this with a quick dash of Aperol or Campari for color and to back up the grapefruit flavor.