I recently updated my older CX-5 to a new 2018 GT. I LOVE it as well. I also got the cargo floor mat and the allweather mats front and back.
An additional purchase I negotiated into the deal was the Mazda trailer hitch and wiring, not particularly for towing, but I inserted a Bully Bar step hitch, for aiding in cleaning the rear of the roof and more importantly to keep other idiots from damaging the rear of my vehicle if they should bump in to me in a parking lot.
Maybe most important for me was to get PPF(paint protection film) installed on the front, leading edge of the hood, mirrors, A-pillars and leading edge of the roof(up to the sun roof). If you get the PPF I would strongly suggest that you have the front half(at a minimum) of your CX-5 clayed to remove all those contaminants built up on your CX-5 during transport from the factory, then get the PPF installed, then finally rub on and buff on a good paint sealant. Now, I added the OEM brushed nickel rear bumper protector strip but many do PPF instead.
If you need winter tires in the Pacific NW be sure to only get 17" rims for them as 17" tires are a LOT cheaper than 19" tires! My dealer sold me a new set of 17" alloy rims to fit the old CX-7 which they were delighted to get rid of at cost. BTW, new rims will need TPMS sensors installed. If you have your winter roads salted as much as we do here in the Canadian NE be sure to get the plastic and rubber stemmed TPMS sensors, as the more common metal ones will start rusting within a year or two. These plastic ones are what my dealer pointed out as all they use now.
The last thing that I am currently installing are the brighter and whiter replacement LED festoon bulbs for the map, dome and cargo lights. #DE3175 at most auto parts stores.