Lots of under saddle bags out there of different sizes that hangs from the saddle rails......check them out at you LBS....
Sounds like you did not want to go for racks and such, so a small bag under the saddle sounds like what might work for you....but they do have size limits that might not be able to accommodate a full change of clothes and a notebook.... I know you want to keep weight and size down but you can't have your cake and eat it too sometimes....

JMOs

Chombi