Additional aliens
To get more aliens on your save, set the system clock forward by one minute. Load your saved game and walk around for a minute. Save the game, then set system clock back to real time. The bellhop should say that you are a time traveler and that the aliens have launched a full scale attack. When you see the clock, touch it or you will lose all your sanity.

Getting things finished quicker
To get things done quicker, change the internal clock on your DS system. This trick works well if you want to get rooms built faster. Instead of waiting the normal eight hours, just set your clock ahead eight hours. If you need money get guests to stay, set clock forward the desired number of hours or days.