I agree with shmn
. A nandroid backup is the most complete, mirror image backup of OS, apps and settings... but it does require you to be rooted.
In your case, as you are returning the phone, as shmn
states, you would need to root and install a custom recovery and then make your nandroid backup before un-rooting your phone again and resetting the flash counter.
For un-rooted users, the best backup around is Helium
and Helium Premium
from the Play Store.
You can read a review about it on the Phandroid forum...
Helium is an extremely handy app backup solution for Android