Elderflower cordial with sparkling mineral water is lovely. The bottle green range do a nice selection of cordials, I had a pomegranate and elderflower cordial with sparkling mineral water and loads of ice when non driver one day over the holidays. Or, if you like ginger, there are some rather nice non alcoholic ginger beers in bottles, I had a lovely ginger and raspberry. I think the important thing is to make it the same kind of treat that the evening glass of wine is; use a nice glass, or put in lots of ice and a chunk of lime if replacing a gin and tonic.