UK iPhone 5S in USA

Hi all in advance...

So I've got a UK iPhone that is unlocked and will be going to the USA in June for 3 weeks. We'll be traveling to New York, Yellowstone, Vegas, Yosemite & L.A.

The question is... Will I be able to easily pick up a US SIM card on one of the networks to provide me with the odd bit of data I may use or is there sufficient WIFI in these places to cover me?

If I have to get a sim, any preference on the networks and what they offer?

Many thanks