I think 8 on Lanai is too much. Oahu certainly has plenty to offer and has the best beaches overall. Hanauma Bay is a must and Waimea Bay on the north shore is a great swimming beach. The drive in the hills behind Honolulu has some great views and hikes...the rule in Hawaii is drive to the end of the road and walk about 10 more minutes if you want a place that's not crowded. Perfect example of that is Kaena Point at the far northwest, which is about 60-90 minutes on foot from the end of the road. Albatrosses, Hawaiian seals, the place where the current meets and the mountains end...it's quite nice.

Of course, there's also a whole network of touristy things like the Polynesian Cultural Center, glider flights, luaus or anything with promotion in the weeklies. My budgets usually rule that stuff out even if I wanted to do it, but thankfully there's a whole network of more-downscale restaurants, stores, bars, parks and sights, etc., where you see mostly locals but that can be a better experience.