There a lot you can do without doing crunches or other lying-on-the floor work: moves that IMO are more effective anyway. The Ultimate Ball Workout from Supreme 90 has hardly any moves where you are lying on your back, for example.
I'd go with lots of plank work and stability-ball and gliding disc (or paper plate) moves.
For stability ball: roll-ins, pikes, plank with feet on ball, plank with one leg lifted, plank with hands or forearms on ball, rollouts with forearms on ball (either from knees or from toes), 'witches caldron" (plank with forearms on ball, rotating the arms in a circle).
For gliding discs: pikes with feet on discs, mountain climbers with feet on discs, planks with feet on discs and moving one foot sideways and back.
You might also try some standing work: weighted wood chops, or rotating alternating overhead presses, or alternating punches making sure to angle the punches to in front of the opposite shoulder.
Of course, if any of these moves hurt your tailbone, don't do them.