You already have the right idea with the Na Pali hike. If you can devote a hole day to it, try to make it all the way to the top of the waterfall (amazing up there). May be a little wet/muddy/slippery, so bring good footwear. It's also fun to kayak / paddleboard around the Hanalei Bay area, and there are some waterways you can go up.
Waimea Canyon is incredible. May also want to consider taking a helicopter tour of the island - I'm not a fan of helicopters in general, but my wife and I did it a couple of years back and absolutely loved the new perspective.
Po'ipu is fine, but may be more crowded (and I prefer the north coast). If you make it all the way over to the west side of the island, the beaches there are incredible and just feel so remote (but it's quite a hike from where you're staying). There's more than enough to do on the north/east shore.