This small island is one of a series that stretch like a string of pearls just off the north coast of Central Cuba. It's just north of Cayo Coco, one of the biggest and most-developed of these magical isles set in the Caribbeab Sea. Guillermo is apparently named after a local fisherman who guided the young Ernest Hemingway on his many fishing foreys into these emerald green waters in the 1920s and 1930s. Nowadays, Cayo Guillermo is connected by a short briddge to Cayo Coco, which in turn has land connections to the Cuban mainland via a long and narrow causeway across a shallow waterway where thousands of long-necked pink flamingos can be seen feeding. Coco's also connected to most of the rest of the world by means of its new international airport, which makes the dozen or so colorful resorts easily accessible.
My wife and I (from Toronto) thoroughly enjoy the Sol Cayo Guillermo resort and have now spent a week there every other march since 2000. The long, white-sand beach goes on and on and is stunningly beautiful. The palm-strewn grounds are quite exotic and the low-slung two-storey buildings are done in a pleasing Spanish colonial style. The accomodations are very confortable and clean though a bit spartan. The pools sparkle and are very inviting. The food is plentiful though sometimes not up to our usual North American standards. This is offset by the very friendly and accomodating Cuban staff who always aim to please. Day trips are available to visit rowns on the mainland (such as Moron) or to head off-shore for snorkeling/diving around the extensive coral reefs or to do some deep-sea fishing. at night there's live entertainment and a throbbing disco.
If you have the opportunity to visit Cuba, we highly recommend a stay on cayo Guillermo or Cayo Coco.