If you can get away with them, and don't mind looking like a dork, Vibram 5 finger shoes are very light and you could just shove them in the bottom of the bag. The soles are semi-rigid, but the tops are just neoprene. They are also very comfortable when you get used to them.

Don't use compression bags, they wrinkle the **** out of everything.

Everything else can fit pretty easily in a bag. I carry a folio, laptop, change of clothes, lunch, a few toiletries(Deodorant, a travel pack of wipes and some hair gunk) a towel, multi-tool and my gadgets in a large-ish messenger bag without problem. It gets on the heavy side with the laptop, but everything else is easy. Defiantly get an e-reader for space saving, or just use your phone if it's an option(I read on my phone at lunch, and then on my kindle when I'm at home).

The bag is water-resistant, and it's so freaking hot here that rain is just something I ride through(when we get it!)

Edit: I don't carry my helmet around, just stick it on my desk at work. They take up enough space that you're probably going to have to just resign yourself to carrying it separately.