The beautiful white sand beach of Hanalei Bay is a 10 minute drive down the hill from Princeville.
The closest beach that you can walk to from Sylvia's Tower is Wyllie Beach, which is near Anini Beach. Turn left out of the Mauna Kai parking lot and walk to the end of the road.
At the turn-out on the right before the entrance to the hotel grounds, you'll see the sign for the path. It is about a 15 minute walk down the steep, shaded path to Wyllie Beach. (Warning: it will take you longer to walk back UP the path!)
If you walk to your right and cross the small stream, it will take just a few minutes to reach Anini Beach and Anini Beach Park.