I'm a little surprised by how jeans are most people's no-brainer option for heat. Lightweight cotton and linen pants are generally more breathable than denim, especially the kind of 13+ oz denim preferred by most denimheads. Another thing that should be mentioned are Topsiders, sans socks. Just don't plan on doing much walking in them, unless you're a masochist. My no-brainer is jeans because I spend more time outside in the summer, therefore getting more wear in.