You can always mix things up and drink tea, coffee, sugar-free squash (as someone already mentioned) or even sugar-free pop.
I used to be a skeptic about some sugar-free pop as sometimes the taste is completely different however some have kept the taste pretty much the same.
A few things you can still drink are Cola Zero or Diet Cola (this DOES taste different to the full-sugar equivalent)
Dr. Pepper Zero (this is spot on and you would have to take a while to taste the difference to the full-sugar equivalent)
Fanta Zero, most varieties (most are pretty much spot-on but the Orange one does taste different)
Supermarkets also do sugar-free pop, Sainsbury's do a really nice Cherry Zero which actually tastes like fizzy Cherry juice but is Syn-Free.
Just be aware that some of the more traditional pop actually tastes vastly different, such as Dandelion and Burdock, he sugar-free equivalent tastes pretty nasty in my opinion.
As an alternative, if you do drink a lot of Soda-Water, it may be worth investing in a Soda-Stream as you would save a lot of money and plastic waste.
It's a machine which can carbonate water for you so you know EXACTLY what is going into the bottle itself - they also have sugar-free cordials available, including sugar-free Tonic Water, Diet Cola, Diet Lemonade as well as pretty exotic ones such as sugar-free Pink Grapefruit and Sugar-Free Cranberry and Raspberry.
You can buy CO2 cylinders and some cordials from supermarkets like Asda and Sainsbury's but also, the company has its' own site which sell different machines, CO2 cylinders, extra bottles and all of the cordials they make, just in case you can't get the ones you want from your local supermarket.
