Unlock Roller Coaster:
If you type the code 'coaster' (without the quotes) at the 'Redeem' menu you will unlock a large roller coaster.
At the entrance of the park tap xoxoxo on the green present icon to get a free Ghost House.
Unlock Mega Coaster:
If you enter the password 'Wheee' (without the quotes) at the'Redeem' menu you will unlock a Mega Coaster.