Conveniently your choices are FRS and GMRS. A little less convenient is cb, and even less so is ham - particularly the 2 meter & 440 bands. Of the 5 options, GRMS, and the two ham bands require a license and all but cb are FM radios.
FRS is the easiest, they're everywhere. They're inexpensive, require no license and the set of Motorola's on my desk have 22 channels with 121 "privacy codes" so a total of 2662 "channels" to use. The downside is that at .5W output their range is limited and line of sight only. I don't know what the published specs are, but experience with previous FRS radios says about 2 miles max in ideal circumstances. Per FCC regulation the antennas are permanently attached to the radio resulting in walkie-talkies being the only option, no mobiles or base stations.
GRMS radios are FRS radios on (weak) steroids. They have more power though I've no idea how much more. I've also no idea what is involved with the GMRS license, nor how well/often it's use is policed. I am under the impression that channels and privacy codes are much more limited than with the FRS. These have the same FCC antenna rule as the FRS. GMRS also share some, but not all of the FRS channels.
All of the ham rules apply to the ham bands. The entry level Technician license isn't that difficult to get, but it does take a little time. Online practice test here:
http://www.qrz.com/hamtest/ If you go down this path be prepared for a whole new hobby, as that is how everything ham is structured. Even if you just want a pair of HT's (ham-speak for
Handy-
Talkies, or "walkie-talkies") and don't care about or want a new hobby you'll need to spend a little time to figure out what to get. HT's tend to have max power of 5W with lower settings available to conserve battery power and to comply with the rule to use only enough power to make contact. ham radios do not have channels, they operate directly on frequencies.
One of the chief bonus' of 2 Meter ham is repeaters. If you can reach a repeater with your radio your range just grew exponentially. A ham club in Bishop maintains a repeater in the Whites at over 10k', "hit" that repeater with a 5w HT and your broadcast area could be a 75 mile radius!
For "STOP! Don't back over the bbq" type communications I'd go with the FRS. For off-trail hiking and remote excursions I'd go with the 2 Meter HT's.