Same stuff I drink the rest of the year, minus hot chocolate for obvious reasons. I'm just not big on hot beverages in general - I want everything I drink to be super cold, always. Anything carbonated is especially refreshing after being stuck out in the heat though.
I am an avid tea drinker year round, Iced or Hot, Black, Green, Rooibos, Oolong, White, Herbal, mixed or flavoured, even Yellow tea, you name it.... I make my own iced teas - I make a Blueberry Pomegranate White Tea steeped with cinnamon bark, its SOOO good.
I also enjoy the Vanilla Bean Frappacino from Starbucks. Perhaps a little too much. XD Everything in Moderation!!!!
And at McDonalds, I order the Vanilla Chai Tea Iced Frappe with Whipped Cream which is just downright AMAZING when its hot out.
Adult summer drink: hard choice. I recently had a berry sangria that was phenomenal. But the easiest drink for me to make that tastes refreshing is just adding one shot of vodka to a glass of lemonade, or making a drink with half parts sweet wine with half parts lemonade.