For breathtaking views of Danau Sentani, head up to the MacArthur monument on Gunung Ifar. This was where General Douglas MacArthur set up his headquarters after his US forces took Jayapura (then called Hollandia) in April 1944. Today the site is occupied by a small monument and a room with displays on the American and Japanese participation in the fighting.
The 6km road up to the monument starts 700m east of Jl Airport in Sentani. Charter a taksi, or take an ojek (70,000Rp round trip) from the bottom of the access road. You must show your passport at a military checkpoint halfway up, and you may be randomly denied access.