Get all Star, Red Coin, and Flowers in the first 8 levels of a world to unlock special levels. This doesn't have to be done in a single playthrough of a level.
Gold Medal Awards
At the end of every non-fortress and castle level, there is a goal ring made up of flowers and blank spots. Each time you go through a level, each flower spot will be filled in for every flower you collect (up to five) on that run of the level. If after jumping through the goal ring the roulette stops on an activated flower, it turns all activated flowers into medals. Getting enough medals, as below, gets you a number of awards. (Note: The medals do not have to be earned all from different levels: you can get all 30 from a single level if you wish, so long as you play through said level multiple times.)
Unlockable: --- How to Unlock:
1 Extra Life --- Collect 10 medals in a world
3 Extra Lives --- Collect 20 medals in a world
Medals --- The Goal Roulette lands on an activated Flower
Vehicle level --- Collect 30 medals in each world