How about an activity that would help students relate grades, G.P.A. and Academic probation. It might be worthwhile to have students crunch some imaginary numbers, e.g., Mary Sue Studeezmuch has 2 Cs in 3 credit courses, 2 Bs in 3 credit courses, a D in a 4 credit course, and a C in a 1 credit course. What will her G.P.A be, and will she be in any trouble at the end of her first freshman semester? If not, how close is she to getting into trouble? You might give them a number of different imaginary students in difference circumstances.
A related issue could be to create an imaginary student who signed up for a course, stopped attending, and never completed the formal process for dropping the class. Have students figure the G.P.A. for both regular and P/NC grade options in that situation.
Another related issue might be to have them crunch some numbers that show the difference between an F and a 0 averaged into a certain portion of final course grade. Maybe you could ask students to identify in their course descriptions any possible behaviors (or non-behaviors) that would earn a "0."
Have students figure out how many more credit hours they will have to distribute over the rest of the academic year to achieve Sophomore standing. Will she likely be taking a May term?