This stage is Sunshine Seaside (5-1). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
As soon as the course begins, head to the right. Break the sand Goombas on the right to find the first Green Star. You will have to toss a turtle shell at it to break it.
Green Star 2
Walk out towards the water to find a green pipe. Enter it to start a mini game for the green star. Turn on all the tiles to reveal the star.
Green Star 3
Head all the way to the right of the course. Climb up as Cat Mario and save Toad. The happy Toad will then reward you with the green star.
After you star riding on Plessie, through the course you will see a stamp on the right. Time your jumps correctly to get it.
Other Notable Secrets
The first key sits on a pier next to the bugs.
At the location where you can go under with the pipes, go down and grab a secret 1UP.