If you need to carry a mobile phone and some money or perhaps a tube and some levers or even a nic collection of sweeties, we suggest you look at this little bag.
Being Ortlieb it is of course waterproof. There is reflective panel and the bag is closed using a roll and elastic bungee system. All very easy to use.
It is attached to the bike with a quick release mount.
The black is actually more or of a 'slate grey' and the green is really, umm, green. In a good way. It's our favourite.