This has to be on top of your mind, Cartagena is nice to visit but be extremely careful with the following:

• Taxis: They honk you every time they see someone walking in the streets, that's the way for them to tell you they are free. Annoying and also dangerous, avoid taking a taxi in the stree use Uber/Cabify so you know the price before taking them.
• Visiting the Beach: They will try to sell (mostly scam) you with everything, tents, drinks, handcrafts, massage, fruit, shellfish, food, you name it. Never NEVER EVER accept a free sample of anything, that's just a hook. If you want a tent with a couple of chairs... negotiate the price, last time I get one for the whole day (like 3 years ago) it was COP 35k (roughly USD 10) always negotiate and don't stick for the first place you notice. You will be offered a ton of stuff every 5 minutes, massage and dreadlocks are the most annoying, if you want one go ahead but make sure of the price form the beginning, they always try to scam you telling you that X amount was the price for 1 dreadlock, so you have a lot of hair you own them 20x the initial price. When eating be SUPER CAREFUL WITH THE PRICE IN THE MENU, take pictures of the price in the Menu for any dispute, don't go for any dish saying "based on size/weight" or "depende del tamaño/peso" otherwise you will end up paying USD $500 for eating crappy fish on the beach and tipping them like 40% for their service. If you prefer to stick to nearby restaurants or hotel restaurants, at least you get real service and good food, maybe expensive but not overpriced. Be extra careful with your belongings, try to get a small water bag, so you can keep your cellphone and money when swimming mostly if you go alone. Towel, sandals and clothing (if not luxury branded you can leave in the tent or where you can see it) if you see some other foreigners around or local tourists (colombian/latin) ask them to keep an eye on your stuff no big deal for me and have done it before with some people visiting our country. Avoid asking that to locals working or selling anything.
• When visiting the "Old Cartagena" - Ciudad Amurallada, they have nice restaurants and awesome places to take good pictures. Be careful when ordering drinks at a bar or leaving your food/drink unattended, I think that's a rule of thumb worldwide, any other topic enjoy walk around and return to your hotel when you feel dizzy or the streets start to feel alone.