One of the most significant side quests in A Link Between Worlds is collecting the 100 Baby Maiamais scattered across Hyrule and Lorule. These adorable creatures are hidden in various locations, often requiring you to use your merge ability, bombs, or other items to reach them. When you find a Maiamai, simply walk into it to collect it. They emit a distinct sound when you are nearby, so listen carefully.
Once you've collected a certain number of Maiamais, return to Mother Maiamai, who resides in a cave near Link's house in Hyrule. For every 10 Maiamais you bring her, she will upgrade one of your rented or purchased items. These upgrades are permanent and significantly improve the item's effectiveness, such as increasing the Bow's damage or the Lamp's light radius. Collecting all 100 Maiamais is a challenging but highly rewarding endeavor, as it makes your items much more powerful and helps immensely in later dungeons and boss battles.