The easiest way to earn bananas is to do the quests for games. Usually the games are fairly AFK and don't require a lot of play time to get the quests done. Once you have done all of the game quests, it gets more difficult and requires you to do surveys/tasks for bananas.
Downloading apps is an easy way, but sometimes you have to sign up or purchase something.
Surveys are a good way too but sometimes you don't fall within the "target audience" they are looking for (ie you are white, make too much/too little money, don't use the product they are surveying for).