At home, drive to drive image backup the drive, and scrub their files on the image with a file level secure erase tool. After verifying that all your and 3rd party files are intact on the backup, scrub all your (and 3rd party) confidential files on the laptop. Uninstall the backup software and the file level secure erase tool (or better yet run them from live CD in the first place). Make sure the machine boots without errors, and common applications launch correctly.
If you are feeling mean 'salt' it with OSHW designs, FOSS software sources and cat pictures etc. with randomised filenames.
Remove *all* clutter from your laptop bag, only leaving the machine, its PSU and power lead.
In the interests of good will and a swift departure, offer supervised access to the machine in your presence during paid working hours, for them to remove any data they believe belongs to them, on the basis that you will not permit an image copy to be made or for them to retain the physical drive (though if they are willing to pay the full cost of its replacement you will permit its physical destruction in your presence), and that you will demonstrate that the machine boots and runs correctly before they touch it and if it does not boot and run the same afterwards, they are responsible for all costs for a 3rd party repair service to restore it to the operating condition it was in before they touched it + for hire of an equivalent replacement machine while it is being repaired.