One of the worst things you can do with a good employee is ignore their successes. You do not want employee to move on to a new job because you are left high and dry, so you do not praise (publicly or privately) and you do not suggest stretch assignments. Try to do away with that thinking. Instead, figure out how employee's continued success can allow you to grow as well. As he/she grows and develops, he/she can take on higher level tasks, freeing you for stretch assignments in your own career.