I've rented for several weeks on Grenada. Don't leave anything visible inside (that really applies just about anywhere in the world) but you don't need to leave it unlocked. Grenada is relatively safe, there is crime, but where in the world isn't there crime?
The 'main' roads are pretty good really. You'll probably get used to driving on the left quite quickly. I'm a Brit so used to driving on the left but I've driven on the right enough times and in my opinion it only takes minutes to adapt.
You don't really need to rent a car in Grenada. The local 'bus' service will get you almost everywhere, is cheap and really is fun to use.