Bags such as this one are easy to bring along. Maybe not a good choice if you prefer lots of colour choices, but it does come in black and it holds eight litres.
From Wardow
Klättermusen Gaut Messenger bag dark grey
Trekkinn
KlÄttermusen Gaut Crossbody Black
Sneakersnstuff
Klättermusen Gaut Messenger Bag - OS