For best coverage you need a quad-band phone. US GSM uses both 850 & 1900 MHz, similarly to Europe's 900 & 1800 MHz frequencies. I had a nice tri-band phone that I would still be using several years later except the coverage is inadequate so I had to buy a quad band.
There's a range of quads available, including several that have been superceded & can be bought cheap. Motorola seem to have the largest range but there are other manufacturers who have them as well.
As for WiFi access quite a few hotel chains offer access as a courtesy. Hampton Inn & Holiday Inn Select chains both do this.