Elderflower is one of my favourite flavours: I (as a tee-totaller) had several bottles of Bottlegreen Elderflower Pressé to drink (out of champagne glasses, of course) at my wedding!
On topic... Their elderflower cordial is my personal favourite, I find it has the most bite out of the easily available ones (including Belvoir, etc).
(There's a Suffolk ice cream maker (local to my M-I-L) which does a rather lovely gooseberry and elderflower ice cream, and I currently have Coppella Apple and Elderflower juice as my favourite still drink.)
And there's the French café classic - a citron pressé (or better still, a citron vert pressé, available in a Café Rouge near you, er, if you're in the UK) - nothing but freshly squeezed lemon/lime juice and sugar syrup in the water. The only more refreshing thing I know of is an Indian-style lime soda (with the soda at the perfect temperature, so its semi-solidified form comes halfway out the bottle on opening...)
Otherwise, really, I'm more of a juice person than a concentrate person, so not sure I can help that much.