What did the LORD let Moses do from the mountaintop before he died?

Answer: See the Promised Land

The LORD showed Moses the land promised to Abraham, Isaac and Jacob, but told him, "you will not cross into it."

“Then Moses went up from the plains of Moab to Mount Nebo, to the top of Pisgah, which faces Jericho. And the LORD showed him the whole land—from Gilead as far as Dan,”— Deuteronomy 34:1 (BSB)