If in Cartagena, I highly recommend taking a trip to the Volcan de Totumo (a very comfortable mud bath). It's about 35 mins outside of downtown Cartagena, but definitely worth the trip. Find a cab on the street (since cabs called from hotels will most likely charge you more) and embark on your journey. Colombians are extremely friendly and love chatting about their country, and yours, so the friendlier you are, the more likely you are at getting a cheaper rate! Plus, when you get to the volcano, you'll have to strip all your clothes off (except for a bathing suit) and leave your belongings with your cab driver.
The mud volcano is pretty short (about 65 feet high), but is the highest thing in this mini neighborhood of about 10 houses. There aren't many vendors (only a few local huts that sell food and refreshments) so don't plan on spending the day- it takes about an hour. After leaving your belongings (except for your camera, of course) in the cab, head up the Volcan de Totumo by walking up the wooden staircase. A quick walk up and you'll be greeted by 2-3 locals. One of the men will hold on to your camera and take photos/videos for you (he's a pro!), while the other two will be waiting for you in the mud. The mud is a little chilly at first, but feels SO good once you're over the whole mud thing. :) As soon as you dip into the volcano, you'll notice that you float instead of sink while in the mud because the mud is being pushed up from out of the earth. It's an odd feeling being pushed upwards, but the locals will help you until you get the hang of it. Almost as soon as you're in, they'll flip you over and give you a massage. The heavenly mud massage will erase your troubles and stress. They work your entire body and will encourage you to dip your head into the mud as well!
When you've had enough, you must tell your new best friends that you're finished because they'll just keep massaging you. They'll help you de-mudify and send you back down the other side of the volcano toward the lagoon where several female locals will be waiting for you. The women here are not shy and will soon be stripping off your bathing suit and pouring water over your head to get the mud off you. Everybody does it and no one can really see you while sitting half-submerged in the lagoon. It's all part of the adventure!
Once "clean," be sure to tip your massagers, washers, and camera man! They only need a few dollars, but it's rude not to tip. This is what they do for a living. If you want to change clothes before getting back in your cab, there are a few changing rooms that the locals will help you find to the right of the volcano.
Oh yeah, and for the next few days, you may find random dried mud in your ears and/or other places on your body! Be sure to use those q-tips! ENJOY!