A full day trip from Medellin is to visit Guatape and El Penol. An early starts allows you to make the most of the day, and reduce to have a great chance of sun before the typical cloudy afternoons.
A 2 hour drive, takes you to the artificial lakes of El Penol and Guatape. The main attraction is the the huge granite rock (220m high), which takes a total of 702 steps to reach the top and see the panoramic view of the surrounding area (as well as the numerous tourist shops) and the small islands within the lake.
From here you can either get a tuk tuk or walk to the nearby small town of Guatape and see the colorful panels lining the bottom of the homes- called Zocalos, as well as stop for a bite to eat in one of the many plazas or melanin (lake broad walk) before heading back to Medellin.