I've got a rucksack that I've used for 4 years and still in pretty good condition but having carried it around on a school residential trip abroad all week I've realised just how heavy it is. For example, in the evenings when we were just going for dinner I put the first aid stuff in a tote bag and that felt much lighter. There wasn't much more than that in the bag during the day but it was very heavy to carry around all day. I'm looking for recommendations for a lightweight but large capacity bag. My laptop is 17" so it does need to be quite big to carry that and marking etc. I also walk to work so it needs to be waterproof. Ideally I'd like a rucksack but I'm open to a shoulder bag, I just haven't come across one thats big enough for my laptop.