Plantation Token

What token-of-our-appreciation to give to a person who shares knowledge with us about saplings?
We’re a bunch of volunteers who are visiting a sapling nursery/afforestation vicinity, where a person who is the head of corporate social responsibility and responsible for handling the plantation of about half a million saplings, will be educating us on various saplings and will be giving us saplings for free.
We don’t want to let his gesture of knowledge sharing to go un-acknowledged, so what can we gift/give to such a person at the end of our trip?
Our company usually gives gift vouchers to guests, but would this be appropriate? Any suggestions of what we could give him to express our thanks?
Would it be too much if everyone committed to sending a picture diary of the saplings they receive, show how well they grow. That way he’ll know his gift and the knowledge he imparted were truly valued.
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