Holiday Inn Centro Historico certainly looks nice, built in an old colonial mansion. Unfortunately, the rooms aren't entirely practical, with cold tiled floors and tiny bathrooms that get flooded with water every time you turn on the shower.
Nonetheless, it suited our purposes; it was cheap enough to suit our needs and had some excellent room service. Try the pancakes. Whoa.
There's a pool out in the courtyard, although we never took it upon ourselves to venture in for a swim. The views look out onto a busy bus terminal, which isn't as bad as it might appear. With all the activity, you're bound to see something interesting.