When people visit Pearl Harbor, they often just think about going to see the USS Arizona/Pearl Harbor Memorial-- the building that straddles the sunken battleship and allows visitors to catch a glimpse of the noble ship's sad fate. It really is one of America's coolest memorials; it's not just a statue or a plaque, it's a view of the actual ship in its watery grave. It really hammers home how serious the December 7th 1941 attack on Pearl Harbor actually was. But, while you're at Pearl Harbor, there are actually a few really amazing military sites to visit beyond the USS Arizona Memorial. You can actually get an awesome overview of US involvement in WWII as you visit the places where America's role began and ended... and learn about everything that happened in between.
Of course, start with the memorial dedicated to the attack on Pearl Harbor, which brought the US into the war. The view from the memorial is both beautiful and also solemn-- you can look down on the wreckage, and even bring flowers to toss on top of it, or leis to lay on the railings. There's also a shrine with all the names of those killed when the ship went down, and a plaque commemorating the men who escaped; those who survived can elect to have their ashes interred in the ship's wreckage by US Navy Divers. Also keep an eye out for "the tears of the Arizona", the slick of oil from the ship that's slowly leaking and rising to the surface. You can also see the ship's massive anchor (one of two, actually) at the entrance to the visitors' center.
Don't miss out on the Pacific Aviation Museum... it's loaded with some pretty incredible artifacts. Even the building itself, encompassing Hangar 37 and Hangar 79, survived the Pearl Harbor attack and still bears some scars from the event. See a real Japanese Zero that was shot down, a civilian plane that was caught flying through the ambush, an F4F Wildcat, and the Stearman N2S-3 that George H.W. Bush flew. There's also an exhibit on planes from the Korean War, combat flight simulators, and tons more.
If that's still not enough WWII history for you, then make sure to stop by the USS Oklahoma Memorial, which pays tribute to another ship sunk during the Pearl Harbor attack and its 429 crew members-- although the battleship was capsized, it was salvaged and scrapped afterwards, which is why it's so different from the memorial to the Arizona. The orderly white marble standards are meant to imitate the ship's crew manning the rails, which is a fitting tribute to the men lost.
A satisfying end to your tour of Pearl Harbor is a visit to the USS Missouri. In 1999, it was moved from its former home on the West Coast and docked parallel to the Arizona (perpendicular to the memorial). The USS Missouri is significant in that it's where the Japanese surrendered to Douglas MacArthur and Chester Nimitz in Tokyo Bay. People originally were worried that place the Missouri so close to the Arizona would overshadow the Arizona, but the two together have been come to symbolize the beginning and the end of the war. Walking along its decks definitely adds perspective to what it would have been like aboard it (or aboard the Arizona) during the war.
After touring Pearl Harbor and seeing the still-scarred buildings and memorials, it's apparent why December 7th is still, to this era, a day that will live in infamy.