Depending on how long you're going to stay in the US, consider getting a postpaid plan. I went traveling in the US for 6 months around 2 years ago, and easily got myself a postpaid plan with ATT on the first day ($30/mo for 3GB, I think, pricey).
I gave the address of my hotel at the time, and since they couldn't run a credit check on me as a foreigner, they wanted a deposit of $500, which I received back without issue 3 months after canceling the account. I didn't mind the deposit, YMMV. Connectivity was pretty good throughout, a little spotty in national parks and other less populated areas, but fine overall.
Free WiFis exist indeed, but more often than not they come with annoying restrictions and the speed is less than staggering.
As a side note, Americans are being royally screwed by their cellular services providers. In addition to the high prices, in the US they also pay when they GET a call or a text, unless they happen to be on a pricey unlimited plan. This sounds completely insane to anyone outside of the US, where only the caller/sender pays (as logic and basic decency dictate), but it's true.