When he talks about something that can be entitled or recognized as beeing 'great things', he is probably talking about the facts, events or ideas of most important influence, like morals, or, of things with most 'gravitas' in society, so: about things that really affect the life of the people.
The most of people look and interpretate that 'great things' as being an weight in their lives, literally in the negative way; so, when Nietzsche says that we need to speak about those things "cynically and with innocence", he is saying: All of that negative weight, that unecessary and really serious vision about the facts, needs to be complete destroyed by a interpretation, or, by an discourse that literally clean the 'Nihilist' content in the speaking about the 'great things'.
If you want to achieve the Active Nihilism, you need to learn how to transvaluate values (questioning what's the real point with that fixed values -> are they really powerfull, or are they decadent, in a way that disagreggates you phisiology and your psyche creating ressentiment? - That is the process to create your own values) with more facility, and looking at the reality with more stronger eyes and, at the same time, innocent: not letting that the 'great things' of your individual life or of your social and coletive life be an torment to your psyche.
Observation: Sorry for the bad english, i'm from Brazil and not a advanced english speaker.
Observation²: I have a version of the Will to power that explain in details the use of that two words, 'Cynically and Innocence' in that context, but is in brazilian portuguese. When i get more time i can do it for you.