MNL is actually a great place for spotting. I've been there once. Near the Airport, there's a village (I couldn't recall the name). You can get some pretty exciting actions there. You need a local to guide you there though. As mentioned by others, please be careful. There are a lot of squatters in that area.
See the red circle:
Besides, it's also worthwhile to visit the Aerospace Museum of the Philippine Air Force. It's pretty new, opened in June 2007.
Just a quick question, is MNL operating under single runway? (but I saw two runways in the pic O.o )