After you get a letter from Niko in Aboda Village, visit him to get a stamp book. Find the fifteen various stamp stations in different areas that you can visit on the rail map to get a stamp. Return to Niko after getting the indicated number of stamps to unlock the corresponding reward.

Shield of Antiquity: 10 stamps. Note: This shield cannot be eaten by Like-Likes.
Engineer Outfit: 15 stamps.
Sword Scroll 2: 20 stamps. Note: This is the Great Spin Attack.