Required to be in Aircraft :

"valid pilot certificate" (FAR 61.3a)

"photo identification" (same FAR)

"current and appropriate medical certificate" FAR 61.3c this means driver's license if you are flying SLA

"appropriate and current Airworthiness certificate" (or special flight permit) FAR 91.203a (see below)

""effective U.S. registration certificate issued to it's owner" FAR 91.203a (must be current owner...recent emphasis by FAA)

weight and balance info/calculations

current maps and airport information

Documents you can be required to produce (but don't have to be in aircraft)

pilot logbook (be sure to have enough landings logged to make passengers legal)

FAR 91.9 requires "current approved flight manual" (If one was not required by certification, subsequently approved manuals, markings, and placards are required.

aircraft logbooks

Aircraft airworthiness certificate has since be "regulatorily amplified" to mean displayed at cabin entrance so it is visible to passengers and crew"

above provided by legal counsel for AOPA in Dec. 2009 Issue of AOPA Pilot magazine

submitted by Dan Caliendo
Ercoupe Mach 0.14