We stayed at the other Hilton ( not Nordica ). Really nice hotel.
Booked basic room with points/cash. Upgraded to suite. Lounge was offered for diamond, along with breakfast.
We were only there one night though before the road trip. I'm sure that helped with the upgrade, but lounge and breakfast should be guaranteed. The lounge certainly helps cut some costs out of the trip given how crazy expensive food/drinks are in Iceland.