As a US ally in WWII, Manila was bombed flat by the Japanese which is why it doesnt have the old Spanish buildings anymore. The remaining old structures lie in Intramuros and Chinatown. Take a walking tour with Carlos Celdran, which is the best way to see the old historical city.
If you are in CBD area in Makati on the weekends, check out the Saturday market at Salcedo Park and Legazpi Park on Sundays. At the markets you can find fresh seafood and other local delights.
If you have some time, take a few days and book a cheap flight to one of the nearby islands like Boracay or Palawan. If you don't have time for that, take a drive to Taal Lake (56 km away from Manila).