The short version
The wall is free. That's the answer most people are looking for, and it's the best thing on this list. Everything else below either costs nothing or costs the price of a cocada.
We get asked this a lot, usually by people who've just found out what a boat to the Rosario Islands costs. Cartagena can empty your wallet fast if you let it, but the Old Town itself, the part we live in, is mostly free. Here's what we'd actually send you to do, in the order we'd do it.
Walk on top of the city walls
Free, always, with no ticket and no gate and no closing time. Las Murallas are a public walkway, which is the thing visitors don't quite believe until they're up there. The simplest way up from our corner is at Las Bovedas: walk north out of Plaza de San Diego and you'll hit it in a few minutes. Get on top, turn left with the sea on your right, and keep going as long as you're enjoying it.
Do it before nine in the morning or after four in the afternoon. In the middle of the day the stone holds the heat and there's no shade up there at all, which is a fact rather than a complaint. Early, you'll have long stretches of it to yourself.
Las Bovedas, for the masonry and not the shopping
Twenty-three vaulted arches built into the wall between the forts of Santa Clara and Santa Catalina. They were dug in as storage vaults and used as dungeons, and today each arch is a souvenir shop selling more or less the same hammocks and hats as the arch next door.
The shopping is fine. The arches are the reason to go. Walk the length of the colonnade, look up at how thick the masonry is, then use the stairs here to get onto the wall. Ten minutes, no entry fee, and you can haggle if you do want a hat.
The Museo del Oro Zenu
Free entry, on Plaza de Bolivar, and it's the best hour of air conditioning available in the walled city. The collection is Zenu goldwork, small and properly lit, and most people are through the circuit in about 45 minutes.
Go when the heat peaks, roughly between eleven and two, which is when you want to be indoors anyway. Its opening hours have shifted more than once, so check the Banco de la Republica listing the night before instead of turning up hopeful.
Sit in Plaza de Bolivar, specifically for the trees
This is the shadiest plaza inside the walls, and shade is a genuine resource here. That's the recommendation: a bench under the trees, fifteen minutes, no purchase required. It's directly outside the gold museum, so the two go together.
We say this to guests who arrive determined to see everything in a day. You won't. Sitting down for a quarter of an hour in the shade is not time lost, it's the thing that makes the second half of the day survivable.
Plaza de los Coches, and the cheapest snack in the walls
Just inside the Torre del Reloj, the yellow clock-tower gate, there's a triangular plaza with a statue of Pedro de Heredia in the middle. This was the colonial slave market. There's a plaque; read it before you buy anything, because the plaza is now lined with candy stalls and the contrast is the point.
The stalls are the Portal de los Dulces. Cocadas, bocadillos, tamarind balls, a few thousand pesos each. Not free, but close enough, and the vendors expect a bit of haggling. Buy the coconut ones.
Look for the sloths in Parque del Centenario
Straight out of the walls at the Getsemani end, and it costs nothing. There are sloths in the trees, and people will spot them for you before you've worked out where to look. Some will want a tip for it, which is a fair trade for finding a sloth.
Give it twenty minutes on your way into Getsemani rather than making a special trip. It's a park on a busy road, not a sanctuary, and it's better as a stop than a destination.
Plaza de San Diego at 7:30 in the morning
Our square, and the free thing we'd recommend hardest, which admittedly is easy for us to say since we're on it. In the evening it's full: every restaurant around it puts tables out and the noise carries. At 7:30am it's empty, the light comes in low over the rooftops, and it's just people going to work.
The evening version costs you a dinner. The morning version costs you an alarm.
What we'd skip
The horse carriages. They're expensive, the animals work in serious heat, and the route is one you can walk in twenty minutes yourself.
Also: anyone who approaches you in a plaza with a free bracelet, a free shot, or a free anything. Very little offered to you on the street here is actually free. Walking the walls is.
How this fits a day
Morning on the wall, late morning in the gold museum while the sun is worst, afternoon indoors or in the water, plaza at dusk. That's the rhythm here, and almost none of it has a ticket price.
If you're staying with us, breakfast is included when you book directly, which quietly removes the most expensive meal of the tourist day. And when you've walked yourself into the ground, come up to the rooftop. It's open till midnight, and it gets loud after dark, which is the point.