View Your Current Ending Sequence - Hold R + L + Start + Select immediately after the lights go out when done writing in the diary. You must hold down the buttons the entire time. Eventually you will see the ending sequence that would be given to you if the game ended at your current level of progress.

Unlockabless

Clock - Clock Get the first house extension before the first Summer ends.

Secret Items - These unlockables are unlocked when playing through the game and performing certain tasks.

"Money" Tree Have over 20000G and buy it from the Hawker in the fall
Magic Bean sell a cow; if the hawker comes and offers a bean in exchange for the cow, accept his offer.