Heres a good way (albeit cheap) to unlock more activities in a single day, rather than waiting for them to unlock over time. All you have to do is turn the game on, do a given activity, shut the game off, remove the game from the DS, then put the date up one day. Put the game back in the DS, unlock and play the next activity, and repeat the process as needed.

If you turn the game on during a holiday (according to your DS units internal clock), you will get a special greeting.