Question: In the final exam of a course, a student has solved some problems in a really ingenious way.
He gets perfect score anyway.
What is the appropriate way to inform him about my appreciation of his solutions?
(Also what if I don't have the occasion to meet him again?)

Answer: 
I know a professor who writes a special note to top 5 students in his class. Something along the line of :

> 
> "congratulations on a great job in my class ABD255 ... specially I appreciate the way you handled assignment 5.". 
> 
> 
> 

He also offers any recommendation letters that the student might need in the future and an open invitation to do master thesis with him. These are some good ways of showing your appreciation as they will likely count towards the student's career.
You already did: you gave him a perfect score.