I have some time here and there, so I'm just gonna add as I have time and will edit this post.. Feel free to visit back here and there to see if I added in anything..
** This by no means is a tell-all.. Kaua'i Tacoma owners can definitely give you better advice, but seeing how no one has responded yet, I'll show you some Aloha and do what I can from memories. This is also not my trying to step on Kauai-guys toes on many of the secret spots to go to. Always be humble where ever you go and respect the land**
** The biggest possible gripe from locals about others is that they do things that they are not supposed to do and get hurt in the process. Obey the signs. If its says don't go, don't go. Locals grew up in many small swimming holes and know where to dive and where not to dive, then a mainlander dives in, gets hurt, sues, then it ruins the spot for everyone. Another example is the sad story about a tourist who "wanted to feel the power of a blowhole".. He sat on the opening and it sucked him, and did him in. Locals would NEVER do that. Don't be that guy**
-Kauai is a beautiful and very laid-back island.. possibly the most laid-back and "slower" than any of the other islands, (not counting Molokai). This is not a bad thing at all, just don't be in a rush and have the fast-driving mentality of a lot of the mainland when you are over there. Be ready for things to close down earlier than you are used to. Just enjoy life and your 10-year anniversary..
-Our place to stay would be closer to the airport.. There is a hotel right on Kauai Beach Dr. that we have stayed at a bunch of times.. Not sure which hotel it is now as they have changed a couple times. We like staying here because it is fairly centrally located.. Unless huge improvements have been made, get ready for TRAFFIC... and again, don't be in the mainland driving mentality. This hotel is nice and you can easily get to Princeville, Hanalei, and Ke'e Beach on one side, and Poipu, Waimea Canyon, and Polihale beach on the other side.
-Some things to do:
-Kayak the Wailua River and small hike to the waterfall. If they are still around, the company meets up at the shell gas station right in town and shuttle from there.
-Take a little longer hike to Hanakapiai Waterfall (use caution).
-If you are a serious hiker, you can hike and camp at Kalalau (rated a high 9 out of 10 - again HUGE CAUTION), but this is a serious but beautiful hike. (We did it 5 years ago with some friends)
-There is even a dinner cruise, or probably a tour that they will go out on the Napali coast. We haven't done the cruise, but we saw Napali coast while first-hand.
-Queens pools in the Kaanapali area.. This is a huge tourist attraction but beautiful. Just don't go where signs says not to go and respect the former pools of royalty.
-Lydgate park is cool for snorkeling, as well as Ke'e beach (not sure what the surf is like at Ke'e in April)
-There is a waterhole that we have been to a couple of times. The first time we went there it was more of a local hangout. The second time we went, companies were taking tours there.. Someone got hurt and they wanted to close it down. Sorry, I'n gonna plead the 5th and not tell you exactly where it is as this is a sore spot for many locals (even me - and I don't even live Kaua'i).
- If you dive, there is a dive shop in the Poipu area that we have used a couple times. If they still have it (and you are an experienced diver) they ever have a drift dive.
- Get a jeep, a bottle of red, and go down with your wife to Polihale beach to watch the sunset. (don't go wild on the sand). Last thing, you get stuck and no one is there to get you out..lol.. (* I am not saying to drink and drive, you are your own man)
-You are kinda on your own here. You just gotta see who has what. Personally, we usually use Costco for our rental cars, Kayak for our airfare, and Kayak again for our hotel. Because locals get a little better hotel rates/discounts, when we island-hop, we call the hotel directly.