The tour hosted by Historic Markers, Inc. (I love their slogan...powered by Key West History since 1513) is incredible for both it's thoroughness and simplicity. You simply go to their website, note the marker numbers and call 305.507.0300. By entering the marker # you get a brief 90 second recorded message detailing the history of that location. As of February 2011 there were 23 markers listed, which is plenty to keep you busy for a few hours of walking or cycling through Key West learning about our history. And the beauty is that it's free...thanks Key West Historic Mark Tour!
My suggestion for a true treat is to hire a pedicab and enjoy a leisurely ride in the open air, sit back and relax. Pedicabs generally charge about $1.50 per minute, but may arrange a special price for you based on a certain length of time. Negotiate! The driver will no doubt add a little bit of color to the tour. But first ask if he/she is familiar with the tour or has conducted similar trips so you do find yourself wandering through Old Town...which could also be fun.