I've lived on Oahu for a little while now (and for what it's worth, I hate it, even as a single 28yo), so feel free to reach out if you have any questions or are looking for recommendations. I'd offer to show you around, but I may well be traveling.
Some hikes you might be able to do are Manoa Falls and Waimea Falls. Both are relatively short. Waimea Falls is fully paved all the way back, but it's all the way up at the North Shore. Manoa Falls is pretty straightforward and well-traveled, and a quick cab ride from Waikiki. You can check YouTube for videos of the entire hikes. If you're wife or granddaughter are looking for a workout with a view, tell them to check out Koko Crater trail across the street from Hanauma bay. There's actually a public shooting range that uses Kokohead as it's backstop, but you really don't want to waste your time there. Diamond Head is another hike that offers a good view and is relatively close to Waikiki, but there's quite a bit of elevation to it.
If your granddaughter's looking to surf, you can check out White Plains Beach over in Ewa Beach too. It's a drive, but it never rains in Ewa, so you can go there even if the weather is bad in Waikiki. The big waves start rolling into the North Shore starting in October, so you may not even be able (or want) to swim up there.
In my opinion, the best public beaches for swimming are on the East side of the island - Waimanalu, Lanikai, and Kailua. Depending on when you go, parking can be tough for the latter two. It's a pretty drive that way too with a lot of lookouts to pull off on. If you're so inclined, you can also find kayaking tours that launch off Kailua and Lanikai beaches. There's some beaches on the West side too, but they're quite a ways from Waikiki.
If anyone in your party is feeling adventurous, you can go cliff jumping in Laie, which you may remember from Forgetting Sarah Marshall. There's also the jumping rock at Waimea Bay, but again, if the North Shore is firing, they're not going to let you go up.
As for food, if you're looking for Korean barbeque, Sura isn't too far from the Hilton. Gyu Kaku is a Japanese barbeque place not too far from you either. There is no good American barbeque on the island, period. If you're feeling a little rich, Hy's Steakhouse is pretty good (better than Wolfgang's) and right there in Waikiki. I also highly recommend Duke's, which is right there on the beach - they have good prime rib, but they run out if you go too late into the evening. Orchids is another cool little restaurant on the beach, though I've only ever been there for breakfast. Nico's is a casual place for seafood that's very popular; just upstairs from that is Harbor Court, which is a nice restaurant that has the best french dip I've had anywhere...short cab or uber ride from Waikiki. There's a lot of your usual chain stuff too. Yardhouse is a good place to take your granddaughter for her first (legal) drink. I recommend that you avoid eating anywhere at the Hilton proper; everywhere in Hawaii is going to be expensive, but the Hilton Hawaiian Village has ridiculous markups on food. Take an extra two minutes to walk to anywhere off the property and you'll probably cut your bill in half.
Definitely sign up for HHonors because you'll earn a ton of points (free rooms!), just don't expect any special treatment beyond those points that you accrue. I'm Diamond with Hilton right now, and I don't get treated any different than when I was at the bottom level. I'll probably be a few days shy of Diamond again this year and I'm not going to go out of my way to keep it.
