From what I heard, many dental schools such as UW and UoP like students who play musical instruments such as the guitar. Perhaps you could put that down.
Uhhhhh are you suggesting people put down things they don't actually do?
Bad idea. Especially because I think this type of thing could very well get brought up during the interview.
I think I'd be able to tell whether someone plays my particular instrument just by talking to them. There are things people who actually take part in certain activities are very familiar with that the general public doesn't know that much about.
You don't want to get caught in a lie. That's unethical and would be catastrophically detrimental to your application. So don't lie.
If someone doesn't have anything to add for the manual dexterity section, that person should pick an activity they will enjoy and start doing it.
I don't suggest musical instruments unless someone is actually interested in them, because there can be a steep learning curve depending on the instrument and factors like how much musical talent the person actually has (eg a tone deaf person will have a harder time figuring out when he/she is in or out of tune).
Arts and crafts activities seem like the easiest way to go here. Building models, sculpting, sewing, painting, etc.
If someone is into technology, maybe something like making robots would suit their interests better. My boyfriend does robotics and doing things with small parts (like soldering) definitely develops fine motor skills and coordination.
Again, for the people who do research, things like pipetting, plating, etc are great too.
Just write about something for this section. There is plenty to use, so no one should be making things up. You don't want to get caught in a lie.