Have a Sims 2 Game Boy Advance game in the cartridge slot to unlock Mongoo Monkey in the Casino.
Want to fight more aliens? Set your system clock ahead one minute. Load your game and walk around for a minute, then set the system clock back to normal time. The bellhop should tell you you're a time traveler, and the aliens have launched a full scale attack. Make sure to touch the clock as soon as it appears, or you'll lose all your sanity.
First, go to the desert with a metal detector. Find the crashed spaceship and use the detector there to get a mummified alien. Now, buy the Government Lab for 2,000 simoleons. Get near to the table and select 'alien autopsy'.
Thanks to Revolution Reader gavin and buki!