Postcards

You can complete a small side quest in Traverse Town. Ten Postcards lie hidden all over the town. Find and mail them by dropping them into the First District Mailbox to receive rewards.

Lockbox in a wall
Use blue trinity by shop - it's in chest up on balcony
In clock in gizmo shop when powered on
In clock in gizmo shop when powered on (second one)
Use box to get on top of accessory shop, in chest
In item shop, attack the ceiling fan a few times
Gepetto's House (after you defeat Monstro)
Head across the rooftops from the Gizmo shop into the 3rd district, and go to the corner of the balcony
Going from 1st to 2nd district, turn right, high jump to awning
Go into the synthesis shop and examine the poster on the wall to the left of the fireplace.

For most of these you get little things like mythril and elixirs. 7 is Megalixir, 8 is Orichalcum, 9 is defense up and 10 is power up.