You could do as Jona suggests:
1. Create the NEW user account (administrative) for your wife
2. Log into the new account
3. Now, DELETE the old account (YOUR former account)
The Mac will be "as it was before", with the exception that your old account will be gone.
If you want to be "squeaky clean" about things, you could do it this way:
1. MAKE SURE YOU HAVE A BACKUP FIRST (I always recommend Carbon Copy Cloner or SuperDuper)
2. Boot to INTERNET recovery (command-OPTION-R)
3. Use disk utility to ERASE the internal drive (make sure you go to the view menu and choose "show all disks", so that you can see the internal drive)
4. Re-install the OS using the installer
5. When you do the initial setup, have your wife create the new account (this automatically will grant her administrative privileges).
6. After the initial setup is complete -- if you have 3rd party applications you want her to keep using, use migration assistant to migrate "apps only" (DO NOT migrate your old account).
NOTE: doing it the first way retains the version of the OS that you're using now.
Doing it the second way will install the latest version of the OS that's available for the Mac you have.