Five symbol stones are randomly throughout a level each day. If find all five of them you will earn a 500 coin bonus. You can find a set of symbol stones once per day, and if done consecutively, the coin bonus will increase.
When you encounter a large shiny question mark, jump into it. You will be sent to a bonus room and end up on a golden railway. As your speed increases, collect the coins and lucky eggs. Lucky eggs give you a chance to get mythical objects that help unlock more characters.