It offers great views of Honolulu, Diamond Head Crater and Manoa Valley and it's also a good workout. It should take 3-4 hours (depending on the pace).
To get to Aihualama trail follow the Manoa Road into the back of Manoa Valley. Manoa Road ends at Paradise Park, continue up the road, through the parking lot, towards the entrance of Lyon Arboretum. At the sharp left bend in the road to Lyon, proceed straight ahead until you reach the parking area and trail head, you'll have to pay 5$ if you want to park. Hike 0.8 miles on Manoa Falls Trail and you'll reach the base of the Manoa Falls, there is a marked trail that proceeds left, this is Aihualama Trail.
Don't get discouraged about the rough start of the trail, the're a lot of rocks and you'll feel like you took the wrong turn but keep on going. Once you get through that you'll see some banyan trees, great views and a bamboo forest. They're a series of switchbacks, skirting the upper end of Manoa Valley, that will take you up to Pauoa Flats. The trail has really low traffic and it's nice to get away from Monoa Falls cluster of hikers. Once you reach the top you have a bench you can sit on and enjoy the view.
The trail can get really muddy if it rains and because on some places it's rocky and can be slippery, bring good traction shoes.