A lot of people use the FRS radios that can be found in most any store, and don't require a license to operate. These can work well, the problem is that a LOT of people use them, and it may be difficult to find a clear channel. There are also GMRS radios, which cost a bit more, so they're not as common, and they require (a fairly cheap) license to use. Or if you want something that almost nobody will be using, you could get a set of true business-band radios, but those require a more expensive FCC license, and cost about $150 EACH for a good-quality set (so $300 for a pair)