here is an article with various suggestions for how to build floating decks https://farmfoodfamily.com/cheap-floating-deck-ideas/ hope it helps
I would avoid using pallet wood for this. It's not the right dimmensions for a surface that needs to be weight bearing and the types of wood used varies. Pallet wood really needs a solid substrate such as a subfloor or wall before you can install it.
Here is one with a fire pit! https://www.hometalk.com/33068054/pallet-deck-with-fire-pit