When the weather gets this hot no one wants to drink a hot cup of tea, but who said you have to make tea hot?! Cold infusions, though not effective extraction for all herbs, can be really fantastic and markedly different from hot infusions.
Making a cold infusion is as simple as pouring water from the tap (room temperature) over dried herbs in a jar/ pitcher, and putting the it in the fridge overnight. recommend at least 6 hours of steeping time. Strain it into a glass and drink. Some people might like sweetener added, which is fine. Note that most sweeteners are less soluble in cold liquid. I store it in the fridge throughout this process to prevent bacterial proliferation. If not consumed within three days, time to dump it and start a new batch.
Nettle leaf
Nettle makes a deep emerald green cold infusion, especially if left to sit for 12 hours or more in the fridge. Nettle is mineral rich, diuretic and mildly stimulating. It's important to drink water in addition to diuretic teas, so you don't dehydrate yourself. Nettle is also used for hayfever symptoms with a lot of success.
Marshmallow root
Marshmallow is my favorite cold infusion. I drink a quart a day currently, as I am working on rebuilding the lining of my stomach. Marshmallow is great to soothe inflamed GI tissues, rebuild mucosal lining, and moisten the entire system. It is also soothing to inflamed tissues in the kidneys and bladder, so great for recovering from infections of that system, or recent passing of kidney stones. The taste is pretty bland, so I like to combine it with licorice, and sometimes fenugreek for taste, and for their additional soothing actions. Slippery elm can be used for the same things, but since it's more expensive I don't use it much. Drink 1-2 quarts of this a day.
Valerian root
Cold infusions extract very specific things, one of which is very light volatile aromatics. Luckily, in valerian those are the most active bits, making a cold infusion rather sedating. start with a small dose (1 tsp in 12 oz) and work up from there to see what will work for you. Valerian is a strong sedative for sleep and anxiety, so this would be great to drink right before bed.
Roasted Barley
Roasted Barley is a cooling and gentle diuretic. The cold tea is traditionally drank in Japan and Korea during their hot summers to cool the body down. In Japan it's called "mugi cha". Bags can be purchased at most Asian supermarkets, with some actually designed for cold infusion. You can also roast your own! It's often mildly sweetened, though I quite like it by itself. Obviously not gluten free, FYI. See the picture for my favorite brand.
Hibiscus
Hibiscus flowers can be found at both herb stores and Mexican grocery stores (under "jamaica"). The tea turns a deep red-pink, and it quite sour and astringent unless sweetened. You can sweeten it with honey, sugar or stevia. The flowers are cooling to the cardiovascular system, especially great for those with high blood pressure due to microirritations in veins and capillaries. They are also cooling to the urinary tract, having a similar effect as cranberry juice. It makes a great preventative summer tonic if combined with marshmallow and licorice (instead of sugar).
Have fun experimenting!