If you Select a mission rather than playing the main story. Each level has 5 Missions which are more or less the same. These missions will be as follows:
Mission 1: Complete the Level which is the completed mission from Story Mode
Mission 2: Collect 100 Rings
Mission 3: Find the lost Chao
Mission 4: Complete the level in a specified amount of time
Mission 5: Clear Hard Mode
Most of the missions give an A rank based on Score. Only Missions 2 and 3 Rank based on Time. Mission 4 involves completing within a specified time but still ranks on score. If you surpass the time limit the level automatically restarts.
The treasure hunting Levels are likely the easiest to get this on. I recommend Wild Canyon. You will be required to have the Sunglasses and Dig upgrades to complete hard mode.
You can A Rank all the score ones by finding the Gems without hints and quickly. It is a small map.
Rings are easy to gather as well. Just note where all the Ring capsules are located.
The Lost Chao is found in a secret room. What you will want to do is take the wind tunnel up. Once you are in that middle area there is one of those paintings that you can dig in the middle to go to another area. Once through you will be in a room covered with the same paintings. Take the 2nd to the right and Chao is in there. Should take 30 seconds max
Mission 5 is pretty easy since the Gems are always in the same locations. In the Wind Tunnel. You will need to use the sunglasses on one of the platforms with Rings to see a spring which will launch you to it. Another will be under a platform at the top area , it is hard to miss. I believe it is the side with the sphinx holding a box and the last one will be in the same area as where you found the Chao