If United does not through check your bags to your JL flight to Manila, then as soon as you come out of passport control/customs formality (which will be 2nd floor of the terminal), go up to departure check-in area (3rd floor of the terminal) immediately, check-in with JAL and drop off your luggage for your Manila flight, then head out to the city.
What do you want to do in the city? Have a dinner? Look at sites?