Beach is the first level, and will always be there.
Achieve the tunnel by going through the beach once.
Shoot an apple at the Electrode to the right of the place full of screeching Magnemite and it will explode, revealing a new path that leads to the volcano.
To earn the river, keep getting more and more points after unlocking volcano to make Prof. Oak think you are trustworthy to go there.
Near the gate at the end of the river, you will see a switch. Behind the switch there is something small and brown floating. Shoot a pester ball, and it will reveal a Porygon (Though you will not be able to take a picture. If you really want a photo of Porygon, shoot at the other green one just behind you when you reach the Cubone Tree) that stamps on the button, and makes the gate open to reveal a split and earn the cave.
To earn the valley, get enough points required (after unlocking cave).
To earn rainbow cloud (where the only thing to take a picture of is Mew), earn all the signs prof. Oak asks you to collect.