If you're searching for outdoor adventure, make a day trip to Kusenbuyama. No walk around the block, this hike is great for a mid-vacation work out.
When you've finished at this steep landmass in Tosu, why not check out Tosu Premium Outlets, Saga Horse Racetrack, and Frespo Tosu? They're not too far from Kusenbuyama and also popular with many tourists.