There are stickers with bumps on them that provide tactile feedback to users' fingers intended for use on iPhones and iPads. It's a product called "Tactile+Plus" made by a Japanese company called Nosho-An which you can see shown in the following image:

Tactile+Plus image

Reapplicable versions of these stickers that can serve as bumps for the F and J keys. They can also be sold with the computers themselves and let users customize the position of the bumps themselves should they want to use non-standard or different sized virtual keyboards (think of virtual keyboards on Linux etc.)

Instead of the ringtone you usually hear when you dial someone they could easily replace it with audio ads. Targeted just for you of course.