If your going over there for the day, then there is really only time to visit 3-4 "hotspots". We used S'Gening as our choice of fast boat to the island. The boat was nice and clean. The boat is about 500,000 IDR = $36 US for round trip.
As stated by some in reviews, they have not caught up the with the mass level of tourism that has hit this place so to get on to the boat, you will wade through about knee to thigh deep water, walk up a little wood plank and step over the side to enter the back of boat. I didn't mind this, because I like having local authentic experiences over a "tourist" experience. So you will do this again when you head back to Sanur beach.
On the island there is small dock to exit and enter the boat.
I had a private guide, so we opted to rent a couple of scooters to get around the island. if you don't mind the heat and sun exposure, then I would suggest this option. Its fun, a bit scary at times when you're faced with having to pass a large car, and there will be some light "off roading" in one area. However, you can get a around pretty quick and beat the crowds. The roads are pretty newly paved in most all the areas we visited except one.
When you reach the Kelingking Beach, if you are in decent shape for climbing then my suggestion would be to climb down to this beach to swim and skip the Crystal Beach. Take lots of water for the return trip back up, and don't take this climb lightly, it is tough to get out even for the most athletic of people. Not many make the journey down so it much quieter with less tourists.
We went to Crystal beach, had lunch a little stand and then went swimming. The water is a bit rough, the sand is hot and coarse on your feet and If you don't rent a chair and umbrella there is not much in way of relief from the hot sun. We got there too late and all the chairs were taken. Even with the sunscreen, I got burned.
Overall, I enjoyed this day.