Cartagena is a "proper" town, and Panama is so narrow, wevgot a taxi to take us the 50 miles or so over to Panama City via The Montefiori Locks, we took in about 8 countries that we wouldn't have normally get a chance to visit.
Recently I looked into tourism in El Salvador just for curiosity...
We're you working the cruise ships? I went to all these places with Ocean Village 10 years ago. Very much enjoyed Belize, Progresso, Routan and Cartagena (Colombia) and the Panama Canal.
I can't give the OP any tips though as I was only there for a day a piece (better to have loved and lost...