If you have an iPhone, you can use the T-Mobile test drive app to get 30 days free. You do need to be in the U.S. or VPN there to set it up, but you don't need to pay. Download the app in advance and set it up on the wifi at your arrival airport.

I think (but don't remember 100%) that Visible's 14-day trial works without a card. Visible or Mint should also let you pay for your service with a foreign card.