Don't worry about malaria. Luzon and most of the Philippines are low risk areas. In all the years I've been travelling to and living in the Philippines, I've never taken any anti-malaria drugs. Also, those drugs can have severe side effects.
As for travelling around Luzon, there is loads to see and do. It just depends what your interests are. If you're into diving, Batangas is great and Subic has wrecks. Tagaytay is nice but can be done as a day trip from Manila. Although some people advise not to bother with Manila, there is lots to do there and it's probably worth a couple of days.
I've never been to the Rice Terraces but I've heard they're impressive if the weather is OK. Baguio is a really nice place and the cooler climate is very pleasant. Vigan is a World Heritage site but I haven't been there. I'd really recommend Ilocos Norte. There loads to see and do there including Laoag, Pagudpud beaches, Paoay Church, Malacanang of the North, Marcos in Batac, Fort Ilocandia, Burgos Lighthouse. Also, the road to Claveria and Aparri is very scenic.
From that area, you could follow the Cagayan Valley, visiting the towns and tourist attractions along the way, before heading up into the mountains and back down towards Manila.
