I use a Levenger International Pocket Briefcase for my wallet: http://www.levenger.com/Cases-15/Pocket-Briefcases-765.aspx.
A Field Notes-sized notebook fits perfectly inside along with business cards, credit cards, cash, etc. and a very slim pen. When closed, it's about the same size as a Pocket Moleskine.

For the pen, I use one of these that I got from Jetpens: http://www.jetpens.com/Ohto-Needle-Point...k-Body/pd/3195.