There must be tons of concentrates, cordials and other things people add to water I've never tried or heard of.
Personal favourites include:
Fresh lemon or lime juice and plenty of sugar.
Ribena (original) - a blackcurrant flavoured cordial packed with sugar.
Calpis concentrate - we get big litre cartons of this. No need to wait until we're in Japan. Great with still or sparkling water.
Elderflower cordial -
recipe here. Absolutely delicious!
And a local product from Sainsbury's (one of our largest grocers) called
High Juice Peach Squash
Main three ingredients are: Sugar, Peach Juice from concentrate (35%) & Peach Puree from concentrate (15%)
These are all bad news for the teeth.
The only teeth friendly one I know of is a sugar free French product I can rarely get hold of called
Antesite
There are two varieties I'm aware of - one is based just on licorice (green)- the other has aniseed added to it (red). Alas, I only learnt about this on my last visit to France when I picked up the aniseed loaded one by mistake
It's one of the few concentrates I'd actively recommend for children. I had a playmate/neighbour in Spain who came from a French family and they introduced me to it - there's no problem whatsover with drinking gallons of water laced with this stuff. Licorice is incredibly sweet compared to sugar and a couple of drops of Antesite is all it takes to fragrance and sweeten water, leaves a refreshing and very pleasant 'aftertaste' so that water seems less 'harsh'.