For a start, here is some info on gardens and hikes.
I have heard great things about the
Lyon Arboretum. It has a collection of rare & endangered and exotic plants. It is located in the Manoa Valley (part of Honolulu). It is often a wetter area so make sure to take a raincoat.
There are a few different locations for the various
Honolulu Botanical Gardens, including one close to where you will be staying.
I also found a website called
Oahu Gardens which provides a list of gardens all over Oahu.
I have not been able to find anything specific about orchid gardens (although some of them may be listed on the Oahu Gardens website. I keep forgetting to ask the teacher at my school that raises orchids. Will try to remember tomorrow.
As far as hikes, this
Hawaii Trails Website is excellent. It provides lots of info and covers most hikes on Oahu. Some of the hikes in the box on this website are of the hiking trails in the Makiki Forest Recreation Center. You can pick up a map at the Hawaii Nature Center (or I could mail one to you if you would like) showing the trail system of the Makiki Forest Recreation Area. Here is
a map with directions to the Nature Center. Two of the hikes start right by their parking lot. There are a bunch of great connecting hikes in this area. I used to hike here a lot. I would recommend going during the week. Easy parking. It gets more crowded on the weekend when more people hike. I did two posts you might remember about the Hawaii Nature Center (
stripes and
photos near the entrance of the Nature Center).
An interesting and not too difficult ridge hike is the
Waahila Ridge Trail. The linked article describes it and has a nice slide show of the hike. I have enjoyed doing this hike because the parking is good and there are beautiful views. It is also nice and cool in the parking lot which is a nice way to start and end your hike and I like seeing the tall pine trees which remind me of being in a forest on the mainland.
Waahila Ridge Trail was just one that I found a write up on but there are tons of great hikes. You do need to check about the parking and if you need a permit or if it is a closed hike (some are closed now but still have info in books/on the web) be aware that for some of the hikes, there is pig hunting on Wednesdays and Saturdays. The Hawaii Trails website provides all of this info for each hike.
The person that wrote about hiking Waahili Ridge Trail also wrote about hiking
Mount Olympus. I don't think I have done this hike (used to hike a lot with a group and have forgotten some of the places I have hiked now).
Candi wrote about the
Makapuu Lighthouse hike. It is paved and hot in the summer but may be a good hike on a cool winter day and you just might see whales in the water from this lookout area. From where you are staying, you might want to drive down along the coast to reach this hike. She also wrote about the
Portlock spitting cave which I have not seen personally but have heard about.
I also found
this website that shows the location of many hikes on the island and also provides mile info for each hike.
Let the planning begin! 
for no rain while you are here.
) I remember that the views were amazing! 
