Replace the battery of a 3rd Generation iPod (3G)
Difficulty level: medium
This operation will void the warranty (which is supposed to be expired, on a 3G)

Replace the Hard Drive of a 4G iPod (4th generation iPod)
Picture guide.
Difficulty level: medium.
Please note: this operation could void warranty.

Replace the broken display of a 4G iPod (4th generation iPod)
Picture guide
Difficulty level: high.
Note: this operation would void warranty, obviously.