It's definately worth planning your time carefully. Plan to arrive at the airport 2-3 hrs prior to your flight. Consider the traffic from wherever you are.
You can probably leave your bags in a secure room at the hotel until your ready, so thats an option.
If you do want to get some rest, you always have the 'Manila Airport Hotel' next to terminal 1, which do day rooms for a fairly reasonable rate.
Mall of Asia is nearby, and is a good place to mooch. Greenbelt, Makati is another option (but again leave an hour for getting to the airport).
