If you're taking the cross-island lane it's possible to make a short detour half-way along that leads up and up to a weather station at 324m altitude. The station, founded in 1940, is not an attraction in itself but its setting, above the treeline, affords a wide series of views.
Some maps show a trail continuing around 1km beyond the weather station to the top of Mt Shantian (山田山, 437m), but that is a very tough trek for which you'll need a guide and machete.