Gibraltar in one day is totally enough. Gibraltar in one day is pure curiosity.
A small piece of United Kingdom in Spain. A mix of English and Spanish language, cuisine, people… A city on a rock. A little bit less than 7 km2 and a little bit more than 32k in population. You can walk from the northern to southern side in about hour and a half.
There are not many reasons to go to Gibraltar except curiosity ;) There are two main ways to get into Gibraltar — flight from UK or just walk/drive from Spain. The border is purely formal. I was looking for somebody to stamp my passport while entering into Spain for about half an hour.
I think airport might be considering a first sight of the Gibraltar. First is that it does not fit into Gibraltar and slightly goes into the sea. Second is that after landing you have to cross the runway on foot to actually get into Gibraltar ;)
Other sights are Upper Rock, Europa Point, local monkeys and just narrow Spanish-looking streets. The Rock is a natural reserve, which charges either 1 euro for entrance or 13 euro for full pass to all sights in there. Sights include different war-time tunnels, batteries and caves. Another sight at the Rock are monkeys which get pretty aggressive when they see food in your hands ;)
Europa Point is at the southernmost side of Gibraltar. It is the place where Atlantic meets Mediterranean Sea. During good weather you even can see Africa from this point.
Other than abovementioned sights you can just walk around and enjoy strange mix of English pubs in Spanish streets… Most part of the city sits at the foot of the Rock and consist of a web of narrow streets going up and down… Writing an itinerary for this trip does not make any sense as you can explore whole place during one day just walking chaotically ;)