Now lets assume you don't have a gun on you.
Option 1) Outrun it: read this
http://www.infoplease.com/ipa/A0004737.htmlThose guys run as fast as 50 mph, so running is not an option.
Option 2) Climb on a tree: Are you kidding me? Those mofos are distant cousins of my housecat. They outclimb you any day easily.
Option 3) Fight it: lions weigh up to 500+ lbs. plus they are damn good fighters. thats why they are classified as PREDATORS. so guess what? fighting isn't an option either.
Option 4) Lay on the ground calmly and wait for help to arrive or the animal to loose interest in you: This is IMHO by far the most promising approach to handle that kind of situation.... but you never know.
btw: having lived in africa i know that most lions aren't man eaters. which also increases the chances of getting away with option 4, namely not to provoke the animal.