What is the best way to build a fire pit?

We have a preexisting concrete slab that we would like to turn into a fire pit seating area. We have already decided to go the easy route and stack slabs of stones in a square or circle as apposed to using mortar to bind stones together. We might even just stack bricks or cinder blocks. Our main concern is to not burn the surface of the concrete slab the pit will sit on. Any ideas? I am totally open to any suggestion on how to build a fun economical fire pit for my family to enjoy! Thanks in advance!