The Koko Crater Botanical Garden is a nice excuse to get away from the beach and stretch your legs a bit if you're driving around the east end of the island.
The botanical garden is completely contained within Koko Crater, the crater of an extinct volcano, and has a 2 mile path which leads through a number of native and exotic plants and trees. The highlight for me was the large number and variety of plumeria trees planted in the outer crater.
It takes just two or three minutes to get to the garden from the blow hole or Sandy Beach and admission is free. We went on a Saturday morning and only saw a few other people during our visit so it's a pretty peaceful experience.
The website below contains a map and directions (including how to get there on the bus) as well as other details.
Pu'u O Mahuka Heiau State Monument is rarely visited by visitors to Oahu. Most locals of the islands couldnt tell you where it is.
A Heiau is a place of worship, sacrifice, offerings to the gods, a sacred temple. This Heiau is the biggest on Oahu and was believed to be built somwhere in the 1600s.
It is located above Waimea Bay on the North Shore of Oahu, and is a State protected monument.
To find this Heiau, take Kamehameha highway South from Haleiwa, on the North Shore of Oahu. Take a right on Pupukea Rd. (Foodland Grocery Store on corner) and follow the road up the hill. Once up the hill you will see the signs for Pu'u O Mahuka Heiau State Monument. Take a right on Pu'u O Makuka and follow it til it ends, and you are there.
This is another hidden gem located in the mountains behind Honolulu/Waikiki. If you are tired and just want a place to get away from it all (not too far away mind you) then a hike to Manoa Falls my be what you need. This is a cool, quiet and fragrant place that is full of ginger and tropical blooms. About a mile up from the start of the trail, you will reach Manoa Falls (which is fed by 160 to 200 inches of rain/year) where you can take awesome pictures as well as take a dip in the pool beneath the falls...just what you need after a great workout. To get here, find Manoa Road and go north (you'll pass the University of Hawaii/Manoa campus) and at the end of the road, park at the Lynn Arboretum and follow the trail to the falls.