<u><em>To give </em></u>is one of the best things people can do. <em><u>Volunteering</u></em> their time showed what caring people they are.
Explanation:
Gerunds are the words that are in the "-ing" form of the verb but acts as a noun. On the other hand, infinitives are the verbs that are preceded by "to" but do not act as a verb in the sentence.
In the given sentence, the word "giving" is a gerund while "to volunteer" is an infinitive. Changing these two into an infinitive and a gerund will make the sentence as
<u><em>To give </em></u>is one of the best things people can do. <em><u>Volunteering</u></em> their time showed what caring people they are.
Dill's father is always sending him away to visit his aunt and Jem & Scout's father is loving and caring. He doesn't send them away and cares for them very much.