However (by the way hi Deano) should you decide to stay in the north, you are only about 5 hours drive from Bagiuo, a wonderfully Spanish style city nestling on a plateau high up in the mountains just north of La Union. Stay there and have a look about for the day, get a hotel and arrange a day trip to the Benaue Rice terraces, its a very impressive sight.
Alterntively you are about 9-10 hours drive from Tagatay (1 hour south of Manilla), and Lake Taal - you can order your meal at a restaurant on the shores of Lake Taal then catch a ride in a boat to Tagatay volcano, horse ride up to the crater - and as long as the seismic activity is low at the moment swim in the crater lake, which i believe is a very healthy thing to do.
Or, if you just want another beach try Subic Bay. The beaches there are as nice as anywhere I have been to in the south.
But if your really adventurous do what we did. Catch a plane to Vietnam from Clark Air Base and watch the sun set over the Mekong Delta amongst hundreds of other tourists and Buddist worshippers. Its a 3 hour flight and costs 70 pounds RETURN.
I will be over in PI next year and plan to do more in the South this time, but for the past 3 years I have travelled extensively in the North, and often marvel at the biodiversity of Luzon. Good luck wherever you go though, just remember relax and take it easy.