Additional characters can be appear through the following methods. Complete quests to get experience to level up. As you gain levels, you will unlock more character houses. You can also gain experience by completing the wishes of various characters. Save jellyfish jelly by completing quests and collecting the daily rewards. Some characters can only be purchased with jellyfish jelly.
Gardens
Always have a garden planted to produce ingredients for food.
Patchy's Bikini Bottom
You can visit Patchy's Bikini Bottom after gaining some levels. Once there, search for buried treasure chests that are worth about 100 coins each.
Tasks
Tap on idle characters to get a list of tasks they can complete. When done, you will earn experience and coins. Choose tasks based on how long you will be playing the game. Some require a few minutes, but others may require a day.
Thought bubbles
When characters move around Bikini Bottom, thought bubbles will appear over their heads. Inside the bubble is a random item. If give that item to the character, you will earn experience points and coins. Characters will only wish for completed food items, for example, Krabby Patties or Salads. Try to keep some completed food items ready in order to give them to characters that wish for them.