I try to do this every semester, but sometimes I forget and then I kick myself later!
Anyway, you can reward your top students for a long semester of hard work by excusing them from the final exam and giving them an "A+" (which they would have gotten on the final exam anyway). I usually choose 1 or 2 students for the classes under 20 and maybe 2 or 3 in the classes bigger than 20.
I base it on the following criteria:
1. Grades (must be the highest in the class).
2. Attendance (must be perfect).
3. Homework (must have done all of it).
4. Attitude (must be cooperative and enthusiastic in class).
I don't tell my classes of this possibility during the semester, they only find out on the last day of class when I tell the one or two students. I do this as a way to reward the students who are just good students without any obvious motivation. And it's also easier on me, as I have about 15 less students (9 classes x 1 or 2) that I need to administer tests to and these students would get an A+ anyway. Works for me, works for them!