These are the freshly picked leaves (the top one is dried) of a lemon scented myrtle or Backhousia citriodora. Crush them between your fingers and smell the strong lemon scent it's name describes!
I have been drinking infused lemon myrtle leaves for ages, bought from the supermarket in packets! I never knew it just grew...on trees :S silly me. Anyway, I'll never buy it again. All you do is pick a few leaves (I usually use two or three per cup) crush them up and put in your tea infuser or teapot as you would regular tea leaves, and infuse until it's as strong as you like! I now know of a few big trees I can sneak some leaves off discreetly...however, when I have a house with a yard, the first tree I'll plant will be a Backhousia citriodora, unless one happens to already be there!
I always drank this because I liked the taste. I didn't know of the health benefits it also has! This website explains well.
It apparently helps relieve cramps, spasms, headaches and fevers, strengthen the immune system and fight cellulite, of all things! You can have it as a tea hot or cold, crush it and add to foods as a dried herb for a lemony flavour...I especially like the adding to your water bottle suggestion. Plain water gets boring.
I usually just leave the leaves out on a shelf and use them as I want them. I could put them into an air tight container once they are fully dry but I never bother, though I might now as it's getting hotter and more humid.