First, you should know it is already possible for another User to complete the Activities of someone on vacation, and there is a record of this in the Chatter:

In Developer Mode, you can visit Technical --> Discuss --> Activities.

From here you can group Activities by Type or by Assigned To (or both) to get a full list of Activities waiting for action.

You then select them and export them, and can update the name of the Employee and re-import to have them delegated to another Employee:

Be sure to check the box next to I want to update data (import-compatible export) on the Export dialog.