Let me throw out some examples, just to get the ball rolling:
1. One way halls/stairs on the ships
2. Designated hours that you can use your balcony (to prevent people on their balconies on either side of you, above or below you)
3. Mandatory vitamin D/zinc supplements
4. BMI limits
5. Elevator use by exception only, with sign up times, etc.
6. Quarantine times before and after embarkation
7. No daily cabin cleaning/turn down service
8. Sign-up times for shuffleboard, miniature golf, ping pong, or basketballs
9. No one on upper decks during breezy conditions, since germs could carry further, faster
10. Confinement to quarters during certain times of day
11. No childrens' clubs
12. Two meals per day, to accommodate more distanced dining with an option for an additional charge room service for a third meal
13. Lottery system for shows and movies with a guarantee of one show per passenger