After you receive a letter from Niko in Aboda Village, visit him to get a stamp book. Then, find the fifteen various stamp stations in different areas that you can visit on the rail map to get a stamp. Unlock the following rewards by returning to Niko after collecting the corresponding numbers of stamps:
REWARD - STAMPS Shield Of Antiquity - 10 stamps. Note - This shield cannot be eaten by Like-Likes. Engineer Outfit - 15 stamps. Sword Scroll 2 (Great Spin Attack) - 20 stamps.