Plus 1 for Osprey - I have a Synchro 20 which is a superb piece of kit. I regularly have my laptop with me on m 18 mile commute. However, we really need to know the dimensions of your laptop to be able to advise you well. Mine is a 13" Dell XPS, which is tiny, and I don't carry the power supply around either. This means that a 20 litre backpack is quite adequate for the laptop, my clothes and occasionally one or two small items of shopping as well. I have a locker at work so I don't need to carry a towel, trousers or shoes and all of that makes a big difference.