the easiest way to find the crash site is to fast travel to MDPL-13 power station , and find the power lines heading north. Follow the power lines until you pick up the "Recon Craft Theta" radio signal. Then turn facing east following the power lines. When you start taking radiation (1rad/sec) you should be close enough to see the crash site. The alien craft is easier spotted during the day.
At the crash site , is a very powerful alien weapon "alien blaster" and 110 alien power cells. These cells are very rare to find in-game.
Since the crash site is available any time during the game , it makes for a very attractive place to discover at the very beginning of the game , and could be the first destination after reaching megaton. Making the strenght of the weapon extrmely useful for low-level characters allowing you to breeze through the first half of the game.
The radiation coming from the crash site is rather low , and shouldn't become a problem since finding the crash site and the items will be done rather quickly , radiation poisoning shouldn't be a problem.
Dogmeat can be used to retrieve the weapon and ammo.