[Cd]

Welcome !
As an SUV, I really like the styling Toyota has done on your vehicle.
I have seen a few recently in my apartments, but thought they were a Honda.
I even searched through the latest SUVs Honda has to find it.
I was curious about the vehicles Cd, since I was seeing several features, such as the raked rear glass, as well as air curtains.
It was only when I really took a look at it that I saw it was a Toyota.
After looking online, I see that the standard version of your vehicle comes in at .34, and the vehicle is apparently a Prius with an SUV body.
It has the same internals.

The Hybrid version comes in at a fairly impressive .32 drag coefficient.

I think Toyota has done quite a bit of work tweaking the aerodynamics of the vehicle. Removing the spoiler will most likely be worse for drag and also ruin the looks of your vehicle. ( You will have exposed bolt holes and such )
A really easy way to see what is happening back there is to tuft test the rear window.
You are in luck, since you will not need a chase vehicle to see what the tufts are doing.
( all you have to do is look in your rear view mirror at the rear window )
Since the vehicle is an SUV, it is larger and taller than a car.
One modification that will almost certainly lower drag is to lower the vehicle itself.
The modification is reversible too, but you may actually like the look as well !

Other than this modification, your main source of improving your fuel economy is to change your driving style.
There are videos on this site on how to correctly drive a hybrid to maximize fuel economy.
( Your vehicle is a Prius underneath after all )

And as others have mentioned, smaller tires without offroad tread that are aired up to max sidewall pressure will also help the fuel economy.

Good luck !

EDIT : I just noticed the post above me is saying the exact same thing ! ( i didn't read all the posts )