Manukan (as well as 2 other islands) are served by ferry's from the pier (about 4 USD rt from multiple vendors more if u want to pay for your own boat). There is a fee (small) for entry to the island. There are 2 other small islands (forgot their names) which are about 5 or 10 minutes from Manukan (which is the only one with overnight accom.) but it looks pretty primitive and I wouldn't stay there as it is very expensive and pretty uninteresting compared to staying in town.
The other 2 islands served by the day ferry's have not been recommended by people I have met. One b/c the fish bite a lot (which is partly a problem of selling bread to feed them which is also true at Man. but hasn't seemed to cause as much biting), and the other b/c it just wasn't interesting. If u do decide to visit more than one island keep the voucher u get for paying the entry (park) fee b/c it is good for all 3 islands on the same day.
There are some other resort(s) which are private and not served by the day ferry, but I haven't been and can't comment.
If u go to KK don't miss the fish market which is set up each night, loads of fresh fish, produce, etc for sale. But since you're unlikely to want to cook
you'll be interested in the various food stalls selling all sorts of interesting food and multiple stalls selling grilled fish of various sorts including HUGE shrimp. You can get the same thing in a restaurant nearby, prepared to order, but at a much higher cost.