I really like your idea. I think personal appreciation is always better and more meaningful than generic notes. Reflecting on my personal experience, I find myself more motivated when a coworker or my manager appreciates me for my work.
I had a suggestion for your idea: I think that while hand-written cards are beautiful and personal, an email, with the person's manager and corporate upper-management copied may be more more appreciated because it could translate into monetary compensation. In my company, the managers take into account all the kudos their employees receive for end-of-year bonuses.
If the notes are personalized, they will highlight the talents of the employee in context and the possibility of monetary compensation would make them more inclined to work even harder and improve more.
I think this is a great idea. reflecting on my personal experience, some of the the most meaningful conversations I've had with coworkers have been over lunch.
I had a suggestion for your idea: make this a corporate or upper management sponsored event. I believe that if people see that their management cares about them and takes steps to appreciate them, they will be more excited to participate. It also sets a tone for the company culture, where people are inclined to mirror the behavior of their superiors and appreciate their coworkers.