I've spent a lot of time in many different areas of the Philippines and a little time in Borneo (the small country of Brunei for a short work trip).
I have the same interests as you, and the Philippines is an amazingly beautiful country. The people are so kind and generous. On more than one occasion I've eaten in someone's humble home and had a great meal and made many new friends. And I offered to pay for the food, some will accept and some won't.
I haven't been to Palawan because it can be pricey to get there ... plane, then taxi, then maybe a boat ride depending on where you stay. I've mostly traveled around Luzon and some areas further south.
In Manila, I prefer the Malate area (near the harbor) to Makati. Makati is very nice but no different than NYC, SF, Singapore, or other major cities in that it's very clean, very upscale, and very expensive. The Malate area is much more authentic and historic. Much cheaper hotels, restraunts, and bars also. I have eaten "street" food many times - very cheap and very good! I've only been sick once - from a nice sandwhich shop in the mall! I've never felt unsafe either. I second the recommendation for Corrigador Island, that was great.
The buses are pretty cheap. I took a bus 4 hours north for a dive trip and it cost $3. The buses go everywhere too, since that's how 95% of locals get to Manila and back.
Each region has a lot of cool stuff: swim with whale sharks in Legazpi/Bicol (SE of Manila), diving in Batangas (S of Manila), and hiking/forests N of Manila. Browsing Tripadvisor is time well spent.
If you are interested in volunteering, there's no shortage of opportunities. The Philippines is poor and prone to natural disasters like typhoons and mudslides. There are probably some formal ways to find opportunities through the embassy or online, but you could also just walk into a school once you are outside Manila. They won't turn you away!
In Borneo, I didn't have much time to explore Brunei. But some friends have been to Kota Kinabalu and loved it!
Have fun!
