Let them use a magnifying glass to examine objects more closely.
Setting fire to a tree stump one lunchtime by testing a magnifying glass.
A normal eye will therefore see an image formed by the magnifying glass most conveniently when it is produced at a great distance, i.e.
red spider mite is just about visible, with the help of a magnifying glass.
Choose something you're comfortable with and that has an easy layout that you can see clearly without a magnifying glass.