Use a small piece of masking tape to write the genotype on the pennies - one letter of the genotype goes on each side of the penny.
I actually did it and that's how it worked out - I didn't make up the data!
For students who are still struggling, have them use the pennies to do other crosses:
RR x rr
RR x RR
rr x rr
and so on
I find that this activity helps students begin to understand what offspring are possible from given parents. It also helps drive home the point that each parent contributes one gene to the offspring.
You could just have them use heads and tails, but marking the genotypes makes it much clearer (especially for struggling students, for whom this activity is the most beneficial).