While everyone is racing to collect upgrade parts (and sending them as Free Gifts) for the new Private Zoo Property...
...I recommend you focus on sending and collecting a different Free Gift: the Engine.
The Engine is needed to build a "Limited Time" Vehicle that popped up in the Chop Shop -- all the way on the left end (the suggestion above to "be on the lookout" makes sense since you usually see new Chop Shop items at the RIGHT end of the row...)
It's called the Patriot and you'll want as many of them as you can get while they're around. With its 53A/68D stats, it is one of the few collectible Vehicles that beats the Foo Fighter on Defense. And you get two Defense stat points for each one you build.
But you'll need 25 Engines just to build your first Patriot. You'll definitely want to use the Zynga/Facebook Double Free Gift Trick to collect more Engines. As noted by a reader, you'll only need 5 Engines for your next Patriot.
Remember - the Private Zoo is here to stay, and requires a lot of upgrading before the Animals have impressive stats. Like the Armory, you're going to have 50 of one upgrade part and zero of the other, and have to trade or beg for the others.
The Patriot, on the other hand, will be here for a "limited time" (until December 23) and immediately enhance your Defense stats.