I'm heading to Hawaii for 2.5 weeks for a course on 3 consecutive weekends. So I have 2 lots of 5 days up my sleeve. On a tightish budget and love adventure, outdoors, surfing, hiking, cycling (and relaxing), can anyone suggest where to stay and what to do? Prefer nature and adventure over seeing historic things. Appreciate the help,
Mel

To appreciate Hawai'i, a bit of history/culture is almost essential. If you're in Honolulu for the course, the Bishop Museum and its waterfront Maritime Center are very helpful. Several botanical gardens explain traditional Hawai'ian agriculture (the Hawaiians did an astonishing job of bringing food plants with them). Then you need to eat a bit of poi.
If you should be based on Oahu, you need to visit one or two of the other islands. Each has special features and opportunities. And probably no place in the world has such excellent guidebooks. The (ahem) sponsors, for example. Also, check out the guides from Wizard Publications.

Assuming you are based in Honolulu, buy the LP guide to Hawaii, find hiking and biking groups (e.g., Sierra Club and many, many others).

If you're fortunate to be there after a rainfall you may catch the waterfalls off the Oahu mountains. The North Shore of Oahu is also very pretty.
Check out the Hawaii Trail & Mountain club web site for listings of their Oahu weekly hikes. Here's some shorter hikes easily accessible by bus:
Diamond Head Crater
Hawaii is all about scenic views, and this is the granddaddy. It's hard to find a more spectacular 360-degree panoramic view of Honolulu and Waikiki Beach than the one from the summit of Mount Leahi, or, as it's locally known, Diamond Head. To savor the sight, take the Diamond Head Crater hiking trail, on the inside of Waikiki's famed landmark volcanic mountain. City buses stop near the road entering the crater. The 0.7-mile trail begins at the crater’s interior parking lot – there’s a small entry fee – heads inside the crater and passes through dry forest to a steep switchback trail on the inside crater wall leading to the upper ridge summit. You then pass through a 300-foot tunnel; the last leg to the summit involves two very steep and long staircases. In addition to the views of the Waikiki Beach area and the city of Honolulu, you'll find sweeping scenics of interior mountain slopes and coastal areas of Oahu. Allow 1½ hours for the round-trip and some time to enjoy the view.
If you finish your hike early Saturday morning, arrive before 9:30 a.m. for the nearby Saturday Farmers’ Market at Kapiolani Community College (4303 Diamond Head Road, 808-848-2074;), and take your pick of lush orchids from the Big Island, fresh corn from Waimanalo, persimmons from Kula (in October) and beans and brews from coffee farms throughout Hawaii. You can have an open-air breakfast (beignets, oat cakes, omelets) and sample local agricultural products — all on the slopes of Diamond Head.
Makiki Loop Trail
Just north of downtown Honolulu is the two-and-a-half-mile Makiki Loop Trail, which affords some of the best views of the city belowl. During the steep walk up the mountain you will appreciate the fragrance of wild ginger, mango, guava, eucalyptus and bamboo. Generations of Hawaiians have told stories of teen-age and other loves sealed or celebrated in the roadside scenic view spots. The trail starts at the Hawaii Nature Center, 2131 Makiki Heights Drive, (808) 955-0100. City bus #17 will drop you off about a 10-minute walk away from the Nature Center.
Makapuu Lighthouse
The bus ride along the Kaiwi coast of Oahu to the island's easternmost point is an Oahu highlight, with views of Koko Crater and the ocean as Kalanianaole Highway curls along the jagged lava coastline. A new scenic lookout at Makapuu has parking and improved access to a trail that leads to the Lighthouse, which has sprawling views of the windward coast — and those luxuriating humpback whales in late fall. On a clear day you can see Molokai and Lanai across the channel, and sometimes Haleakala, the 10,000-foot mountain giant on far-off Maui. At the easternmost stretch, about a half-mile past the Hawaii Kai Golf Course, a gate marks the starting point for a two-mile hike to the Makapuu Lighthouse. The ocean views make this walk notable. The lighthouse, built in 1909 and automated in 1974, sits above Makapuu Beach - famed for body surfing. Seabirds circle the cliffs and tiny offshore islands. Makapuu is a prime spot to view some of the nearly 40 species of migratory shorebirds that begin arriving in September.</UL>

Thank you so much everybody for your time in replying. I'm just so excited about Hawaii now. Those links are fabulous. I hadn't actually thought about which island, the course is in Honolulu, but I actually now think that I don't want to be tied down by that and may go purely for holiday, not work. Sounds like much more fun!
Again, U appreciate your time and keep me posted if you have any other suggestions on any of the islands,
Mel

Do you scuba dive or golf? Can do both of those pretty cheaply on the island. If you have a car, going up on the way to North Shore is a really nice change from Waikiki.
I was in your shoes and put together a few lists of activities -- PM me if you want a copy of my messy notes.

I keep talking to people that miss these three places. You will so much fun you go there.
#1 - Sandies - this beach is on the bottom southern tip 20 minute drive from Waikiki. This the one of the only places that the swells make it to the sand (most other place the reef break it down) You get to see or play in massive waves, the water is dangerous here so only go if you know what you are doing but I always have a good time watching the skilled hit the waves and the dumb tourist get worked over by the waves.
#2 - Pipeline (on north shore)- Go there to admire the pro's rip it up. I would suggest to only do that even if you think you can surf, I thought I could hit pipe line and ended up with a bruised butt and a broken board.
#3 - Turtle Bay (on north shore)- every time I went there I saw at least three turtles and more fish then I see anywhere else.