Going up to Baguio City, there are 3 routes.
(1) Kennon road -- shorter distance from Manila, some waterfalls to see. Buses and big trucks don't use this road (or maybe they're not allowed to pass there), only light vehicles pass here.
(2) Marcos highway -- longer by about 15-20 kms. but the road network is good and less prone to rock slides. Buses and trucks from Manila and Pangasinan take this road. Lots of good sights to see: the flyover, tunnel, rock catchments, cemented mountain sides, ...
(3) Naguillan road -- used by vehicles coming from Ilocos and La Union provinces.
From Baguio and there are some time to spare, take the Marcos highway, pass by Agoo (La Union), see one of the country's cleanest and greenest municipalities! I did not see a single pile of uncollected garbage in that municipality, whether in the town proper or in the barrios surrounding it, during our trip there last March 2005. Roadsides are planted with various tree species, some with flowers, and hills are thick with forest cover. Once you cross the border to the next town, Damortis, the trees on roadsides are either few or absent.
Also, visit Agoo Basilica, more than 400 years old (built 1578 i think). Part of the church has been renovated, but the structure still retains its 4 centuries past.