So I use a Goruck GR1 backpack. They are a bit expensive but the quality and ruggedness are awesome. I use it to carry my items on day's I bike commute to work but I also have used it for business trips when I fly. I also use packing cubes along with it to help organize my stuff but I can easily get a few days worth of clothing and other essentials. Plus it has Mollie webbing that I can attach to if I need to carry additional items. Mine is the 26L version.

I got caught in a downpour a few weeks ago and everything in my pack stayed dry. Its held up very nicely with rough wear, much better than other backpacks I've owned and used.