Hi korshmar, 
The sloth sanctuary is a little south of Cahuita off of the main road so you would need to take the bus from both Cahuita or Puerto Viejo. Puerto Viejo has a lot more going on as far as restaurants, bars, people on the beach and Cahuita is a better place to just relax and enjoy the quiet. If you do decide to stay in Puerto Viejo, Hotel Formentera Playa looks like it is in a quieter spot and people seem to rave about it in the reviews. It is a bit further up the beach but if you are concerned about safety you could just take an inexpensive taxi back at night. That stretch of road is well traveled by both locals and tourists though so if it isn’t too late you might feel ok. It is definitely too far to bike to the Sloth Sanctuary from Puerto Viejo but you can easily catch the bus at the station in town.

Thanks for checking out our site and reading our book! Hope you have a great trip.