1. Take a stroll along the wall of the old city and see all sorts of old Spanish architecture, along with a beautiful sunset! You've probably read all about the old city and it really does live up to it's hype. A majority of backpackers traveling to Colombia from Panama will arrive in Cartagena. It's an amazing introduction to the even more amazing country that is Colombia...
2. Play a game of futbol (soccer) on the outskirts of the walled city. The hostel that I stayed in had a sponsored soccer match every Tuesday and Thursday. Within 15 mins of checking in, I had thrown on a pair of sneakers and we were off. We played on a concrete field with two metal rectangles (no nets) as goals. Three times during the game someone kicked the ball into 3-lane traffic and we had to run into on-coming traffic to rescue the ball. We didn't play with any locals but there were several other games going on right next to us. A few of the girls who didn't want to join us watched from the wall and cheered us on.
3. Explore the nightlife and seemingly endless amounts of restaurants and bars (be warned, this city is expensive) inside the historical city. You'll enjoy beautiful lights in the parks and hanging above the old cobbled roads. Anywhere you walk there are Colombians bombarding you with menus, drink offers, or just about anything else you can think of (yes, this is Colombia). Which brings me to #4...
4. This isn't my thing (only because I don't trust myself), but you can find whatever you are looking for here, within 30 mins of walking around I had been offered everything. One gentleman asked if my group wanted to accompany him to his bar, where he could provide anything, "girls, cocaine, X, anything"...he would continue.
5. CASTLES!! If you like history there is an amazing castillo (castle) that survived at least three raids, in a few different wars. The Castillo San Felipe de Barajas began construction in the 1500's and reminds me of the fortifications in old San Juan, PR for anyone else that has been to Puerto Rico!