To add to the challenge and get your core in on the action, do triceps dips with your hands on an exercise ball. (cathe.com)
You can substitute the full parallel bar dips for tricep dips instead with the feet staying on the floor as an assist. (dieselgrace.com)
Triceps Triceps Dips on Parallel Bars (alternate with Close Grip Bench Press every other workout) 4 sets of 12, 10, 8, 8 reps Straight Bar Triceps Pushdowns (alternate with Overhead Dumbbell Triceps Extensions every other workout) 4 sets of 10 - 12 reps (hugorivera.net)