Lanai is a pretty neat and unusual place, much quieter and peaceful than any of the other islands. A couple of excellent golf courses and a great beach (although can be rough in the winter). You can rent a car (Jeep) and drive to some of the "sites"; we thought Garden of the Gods was interesting, and at Shipwreck Beach, you can walk for miles. But if you are into shopping or entertainment, skip it! Downtown Lanai City is a fun place, but you can check it all out in an hour or so, and that's taking your time.
We traveled to and from Lanai by taking the ferry from Maui, which was fun. One trip was with high winds and sea, so was a bit rocky.