Be required to carry 30 bath towels OR 80 bars of soap? The distance or duration is up to you, but come up with a standard for the class. Also, some Google searches may be necessary to make your decision.
Whichever option you choose, justify your reasoning with mathematics. Maybe you can stack bars of soap in an array that’s easy to carry. Maybe you can load up on bath towels.
This would also be a fun experiment to do in class with bath towels and bars of soap.