The rock aka El Peñón de Guatapé
If you are in Medellin or traveling through Colombia, here is how you can get to Guatape… From Medellin, you can get a bus to Guatape from the Terminal de Norte (Metro stop Caribe.) There are several bus companies that go to Guatape and the average price for one-way is 12,000 COP. There are buses every hour from 6am to 6/7pm and the trip takes about 2 hours. The bus will take you all the way to the edge of the town and drop you off at one street that leads directly into the center plaza of Guatape. There are several inexpensive hostels to stay with the cheapest rooms being from 17,000-20,000 COP per night. There are so many great activities in Guatape. From waterfalls to boat rentals, hiking and fishing, one could easily spend more than a weekend in the town. One of the most popular things to do is the hike to El Peñón de Guatapé that is that big rock in the picture below. 
It is easy to walk to the rock from anywhere in the town. If you take the same road that the bus brought you to Guatape, that will lead you to the rock. There is a walking path off of the road for about half of the hike, but then you will have to continue on the main road for the rest of the walk. Once you have crossed the bridge on the main road, take a left at the gas station, and that will take you up the hill to the entrance of El Peñón de Guatapé. The entire walk to the entrance takes about 30-40 minutes. Or you can take a Tuk Tuk (if you have enough people) for about 2,000-4,000 COP per person. However, the walk is nice and is a good prep for the hike up the rock. 
To climb the stairs (740 steps to the very top) it cost 10,000 COP and it is entirely worth it. It is the only place where you can see the lake and its winding islets. 
Taking a Tuk Tuk back to town is a great end to the day. 
If you are interested in a boat ride, costs will vary with how willing you are to bargain. Our Captain Ivan, who you can find near Lake View Hostel, is a great guy and speaks English. There were 6 of us on the boat for about 5 hours and it costs 18,000 COP per person. He took us around the lake to look at all the houses and cabanas on the islands and then took us to an incredible and private swimming cove. Lake View Hostel is across the bridge down the malecon in Guatape. If you ask there, they should be able to help you find Ivan. He looks like this…. 
Zorba's Caravan 