Just stayed at CY on Kauai at a Plat.

Got room upgrade to the 4th (top) floor with ocean view. Although it's a little dated, it's not too bad.

Got waived the $14 parking fee and free buffet breakfast as a Plat. Both given without asking.

I don't think you can upgrade your cert. To me, CY on Kauai is not too bad, 7 miles north of Kauai Marriott Resort, which I also stayed before. Convenient access to Kapaa and points to the north.