Get to Coba early before all of the tour buses arrive and before it gets too hot. The site is large so consider renting a bicycle for $3. There are places to park them along the way when you want to stop to view the ruins. We did not hire a guide but used the Monclem Ediciones book guide which can be bought at the store that is directly next to the entrance. The book has a blue cover and can be bought for about $6 USD. Stop in the little town just outside of the ruins to have some lunch and to walk along the boardwalk. Then go to the three cenotes that are just past the ruins. Go to all three cenotes because they are all so different. Be sure to bring your swimming suit and go swimming at each one.