If you can try out different materials against your bare feet in the blazing sun, that's the way to go. Brick is pretty warm underfoot. Pennsylvania flagstone (the blue/grey/brown stuff) gets ripping hot. We actually had a client request an irrigation system be retrofitted to spray their pool deck down to cool it off.

Concrete pavers, light colors are obviously better. Travertine needs sealed anyhow, so use a sealer with a little grit mixed in and it'll give you better grip.

Where are you located? Some materials are more readily available in different regions.