Backpack should be placed on the rear rack, not on your back. Make the bike carry the uncomfortable weight. No sweating your back, more comfortable. You can also add panniers if you need more luggage room, but for one day trips - backpack is the most convenient. Just put it on the rear, fasten with bungee strap. I've posted pics of how I do it several times on this forum if you are not sure how to securely fasten it.