Tayrona National Park is must-to-do on your list while traveling in Colombia. Caribbean coast meets dense forest of Sierra Nevada de Santa Marta mountain range and you feel like in a heaven. You´ve got plenty of trails options, many beaches but most of them are dangerous and pretty violent. There are few beaches suitable for swimming (La Piscinita, Piscina or El Cabo) with nice calm water but definitely they are worth it. Be aware that you have to walk to reach any beach you´d like as Tayrona is national park and protected area (it is also prohibited to bring plastic bags, entrance guy can take it away from you).
Tayrona is not just for one day. you HAVE TO stay overnight, you can find campsites and put your tent or pay for hammock. Please do not miss amazing Tayrona sundawns and sunsets!
I visited Tayrona in May 2014 and here are few shots from my trip: