Gamalama's volcanic cone dominates Ternate, rising to 1721m. Major eruptions in 1775 and 1840 wreaked havoc on the island. Gamalama still spews out lava, ash and smoke periodically, but it is possible to climb the peak for stupendous views. It's an eight-hour round-trip and a tough ascent even if you are fit. Don't attempt it in the rain. You'll need a guide too, as the trail isn't marked and the locals don't want you trampling through their farms.
The tourist office in Kota Ternate can arrange a guide (100,000Rp).