Iced tea

You can brew up your own iced tea without the sugar and other nasties added. Caffeinated teas such as black or green are fine, or you might prefer a herbal tea.
I love nettle tea, sometimes with a dash of lemon juice or some sliced ginger – this is packed with iron, and is great if you are low in iron, or get tired around the time of your cycle to boost levels. I also enjoy rooibos, licorice, rosehip, and ginseng based teas. Chamomile or elderflower would be nice options too.
Either way, steep 1 teabag/tspn of the tea in 1L water overnight, then add whatever you fancy from the following!

Slices of citrus – orange/lime/lemon (care with lemon/lime as it can affect tooth enamel in large quantities)
Fresh herbs such as mint, lavender, rosemary or basil.
Slices of ginger work well, or ground cinnamon to infuse.
Sweet things – a dash of vanilla, some stevia, a bit of honey or a little fresh squeezed fruit juice will add some more flavours.
Ice cubes if desired