I used to have VirginMobile... they lease time from Sprint, a CDMA carrier, very reliable. With VM you can buy a phone and then sign up for a plan. On a 'pay as you go' you can choose to pay by the minute, no contract required. You can use for the 2 or 3 weeks you're in the U.S. then cancel the service and throw the phone away, or bronze or whatever...
20cents (100 cents to a US dollar) a minute US or a monthly minutes package. With the minutes package I see they have 200 anytime minutes for $20US or 400 anytime for $30US. Anytime minutes are anytime day/night... as opposed to some packages offer 'no charge' minutes after 9 p.m. until 6 a.m. On their website they have some fairly inexpensive phones, some more costly - depends on what you want or need. You can get texting for $5 a month extra for 200 messages... or more messages for a few bucks more.
You can browse websites like Walmart.com or bestbuy.com to see what sort of pricing they have for the same phones. You can purchase from VM and have the phone shipped or Walmart or Best Buy and have the phone shipped or buy when you get here since you'd need to be in the U.S. for activation any way. Activation is simple, quick, easy, but need a credit card for payment.
There are two types of wireless in the U.S., being CDMA and GSM. Sprint and Verizon are CDMA (includes VirginMobile maybe other small carriers leasing time). ATT, T-Mobile and I don't know what others are GSM.
CDMA is code division multiple access.
GSM is Global system for mobile telecommunication .
For your use you probly shouldn't have a preference. Both cover the populated U.S. about equally for basic phone/texting coverage. The 3G competition between Verizon and ATT really are just about high speed data coverage. Regardless of the cellular company, if you travel outside of the basic coverage area (see their colorful maps) you will be
roaming and your price per minute will skyrocket regardless of time of day or night. Roaming is out of area and the coverage is by some not named local cellular carrier.