Haleiwa on the north shore is a fun spot to get out, walk around watch the waves. Get some of the famous Matsumoto's shaved ice. We rented paddleboards there.
Kualoa Ranch has an interesting tour, including a ride up the mountain to see spots where different movies (like Jurassic Park and Kong vs. Godzilla) were filmed.
I guess the next ones might not be ideal since your wife is pregnant, but...
- We hiked a rainforest just outside Waikiki called Manoa Falls. Totally felt "one with nature".
- My first morning in Oahu took the kids up the Diamondhead crater hike. They were speechless at the view from the top.
- One of the highlights for me was hiking up Koko Head trail. It's right across from Hanauma Bay-- used to be used by the military, I believe. You use railroad ties as steps, and it's quite steep and challenging with an amazing view at the top.