I really like the Kailua side, which is a little less "city" and a bit more chill. Really nice food & beverage scene. Pillbox is an iconic hike in that area.
To me, if on Oahu, this is Hawaii. The Windward Side really is the best place to hang out and it's where all of the movies and TV shows are shot. Kaneohe is a bit of a mess, but Kailua and Lanikai are iconic, as is the non-Kaneohe parts of Kaneohe Bay.
You can rent stuff to go to the Sandbar like Brett said, but you can also rent kayaks to go out to the Mokes off of Kailua.
It really depends on what you want and what you like for the visit. If you're a hiker or swimmer, go to the Windward side. If you want to see views, drive the perimeter. If you are a history buff, look for the museums (Pearl is only one place...there's other cultural centers around, too).
I've been back several times after living there and every time I left, I still felt like I had plenty to do if I came back again.