Cross body bags are a nice midpoint between a backpack size and a fanny pack size and they don't sit on your back causing sweat - I got an adidas one for like $

Cross body bags are a nice midpoint between a backpack size and a fanny pack size and they don't sit on your back causing sweat - I got an adidas one for like $30 and I can easily fit a 32oz water bottle a folded light jacket plus other small things.