Will you have wheels? It's not a very spotter-friendly airport if you don't. However, you can take the underground to Quinta das Conchas station and walk east along Avenida Maria Helena Vieira Silva, past a police station till you get to a cleared site (well it was a cleared site in July when I was there - it may now be a building site). You will now be facing a road that runs along the airport perimeter. Go south a short distance and you will come to a footbridge which crosses the road. This will enable you to get close to the perimeter with a good view of the runway 03 threshold. 03/21 is the main runway so you can get landings or takeoffs depending on which end is in use.
Make the most of the available light ... a lesson of photography that applies to life