I'm going to start bike commuting next month and would love to get some suggestions on a good commuter backpack for my needs! I'll be riding 8 miles each way on a paved multi-use trail. I'd like something waterproof in case I get caught in rain. It'll need to be able to hold my lunch, phone, keys, wallet, dress shoes, pants, shirt, underwear, socks, tie, body wipes, and U-Lock. Something that could double as a backpack for hiking would be great.

And for those of you who do this type of commuting, what do you do with your smelly athletic clothes upon changing? I'd like to avoid cross-contamination between my clean dress clothes and sweaty clothes....

Advice and recommendations appreciated!